Napalm684-
Being that I am not sure where the 50C reading for your mobo is coming from, it is difficult to make any judgement as to the safety of that temperature. However, I cannot think of any component where I would actually be concerned with 50C, especially with a high ambient temp. Perhaps if my HDD were idling at 50C, I would take action, but I assume your HDD temps are not being mislabeled as motherboard temps.
To answer your question about whether you need an aftermarket CPU HSF- if you are hitting 54C at max, I would not be concerned. I would get worried if my processor went over 58C on full load, though, especially if I knew that my ambient temperatures were in danger of getting higher (not an uncommon thing this time of year ). Are you overclocking at all, and if so, how much? What case do you have?
If you do decide to go for an aftermarket CPU cooler, there are better choice, IMO, than Zalman. Nothing against the company, but their CPU heatsinks consistently show mediocre results, with the possible exception of the 9500. They make a hell of a VGA heatsink (VF-900), though.
I would strongly recommend that you look into alternative chipset cooling solutions, though. The newer revisions of the NB fan are better quality and run at lower RPMs, but there have been many reports of even the new fan dying. You have several choices with respect to aftermarket NB coolers, depending on how much you want to spend, how your airflow is, and whether video card clearance will be an issue.
1. Zalman NB-32/47J: Passive solid anodized aluminum heatsinks attached via pushpins, or in the case of the NB-32, you can use thermal epoxy (not recommended). These have been around for some time, and while they do an admirable job as far as passive non-heatpipe based cooling solutions are concerned, there have been reports that they cannot keep up with an NF4 in an overclocked system, unless there is some decent air blowing over/through the heatsink. I have seen temperature readings anywhere from 45C (where a fan was aimed at the heatsink) up to 80C+ in cases with undervolted fans and/or high OCs.
Some users have reported that sticking a 40x10 or 40x20mm fan running at a quiet/silent 5V to the top or side of the NB-32/47 heatsinks drastically dropped their NB temps (fan attached via double-sided tape or screws, or even zip ties). While this would not constitute true passive cooling if you went this route, you would have some peace of mind as you (a) could choose the fan you want to use, (b) easily replace the fan if necessary, and (c) would know that your chipset will probably not burn out (at least for a while, depending) should your fan fail. Call it partially passive if you will.
Video card clearance has been an issue with these coolers, though, as they are somewhat limited in their mounting flexibility, being solid blocks of aluminum and all. The NB-32 is les problematic due to its smaller size; in any event, it is often sufficient to simply bend a few pins to make room. Note that this tends to be a problem more often in systems running SLI with large video cards using large aftermarket (or monster-sized stock) coolers.
Cost: $5-7; 40x10mm fan $5-10
2. Zalman NBF-47: The latest passive solid aluminum NB cooler from Zalman- currently very new, so reviews are lacking. The one review I have come across found that it kept a Gigabyte Intel NF4 at 63C under full load; this was in contrast to the stock passive heatsink, which hit 89C under similar conditions. User reviews are sparse but should be increasing soon. Overall, looks to be a decent improvement over the NB-32 and 47 models, but more reviews are needed to make a final judgement.
Cost: $9-11
3. Chip Force heatpipe cooler: Supposedly one of, if not the best NB cooler out there. Designed to be flexible in mounting orientation, so that clearance should not be an issue. Comes with a 40mm fan, but you can probably get away with passive cooling given some airflow. Reviews claim 40-45C on load, from what I have read, which is VERY low for an NF4.
Cost: $30-35
4. Thermalright HR-05 heatpipe cooler: A larger heatpipe-based cooling solution that looks substantial enough to cool a CPU. Should have no problem handling any NF4, and can be installed in different orientations for clearance purposes. Few reviews, but all seem very promising. Temps similar to the Chip Force, but price is about half.
Cost: $17-20
5. A8N-SLI-Premium heatpipe: The heatpipe cooler that ASUS makes for the A8N-SLI-Premium motherboard is available for sale at the ASUS estore. This is compatible with the A8N and A8N-SLI/SLI-deluxe. ASUS claims that it keeps the NF4 at 70C under full load. From user reports, this seems accurate. Modifications can be made to lower the load temps by 5-10C, if desired. An ideal solution for those who need all the clearance they can get, as the chipset block is very low profile. While not necessarily as effective as active cooling, you will never have to worry about any fans dying ever again.
Be warned that if you use a case where your mobo is flipped upside-down (CPU is at the bottom, GPU and PCI slots are at the top), the A8N heatpipe has been reported to not work very well. I have no personal exerpience, but have read reports where the chipset was not too happy with an inverted mobo; then again, there are people who claim it made no difference whatsoever. Even though heatpipes are supposed to work in varying orientations, this may or may not be a real issue. I believe ASUS recommends against inverting the heatpipe, though.
Cost: $15
There are a handful of other miscellaneous NB coolers out there, but these five are most commonly used for the A8N boards. I personally went with the heatpipe, and am very happy with it. Temps have never exceeded 58C under full load with an FSB of 250, in a case with undervolted fans, thanks to a few simple modifications.
The NF4 chipset is, I believe, rated for up to 90C. Whether this is continuous operation or peak, I am not sure. It is possible you might begin to experience adverse effects at >70-75C.
All of the above require mobo removal, so before you make the jump, be sure that you will be happy with your cooler, unless you really enjoy taking the mobo out repeatedly.