You're welcome, Serpentor.
I think it would be okay to use PC2100 memory in your system. I can run my PC3200 at a variety of different speeds, so I assume that you could run your PC2100 RAM at a variety of speeds as well.
You can change your memory bus speed by percentage increments (rather than simple 1MHz increments). It's kind of complicated, and I don't have the numbers in front of me. I'm going to pull some percentages out of the air to use as examples, but they're not meant to be taken literally, okay? By default, your RAM speed is the same as FSB speed. For example, if your CPU is running with a 166MHz processor bus speed, the RAM speed is also set to 166MHz if you leave the CMOS setting on "Auto". If you change it to "Expert" mode, you can change the RAM speed. Suppose you start with a processor bus speed of 200MHz. The default memory bus speed is 200MHz. In "Expert" mode, you can set the memory bus to run at 50%, 75%, 100%, 125%, or 150%. In other words, you could set the memory to run at 100MHz, 150MHz, 200MHz, 250MHz, or 300MHz. Again, those are not the exact percentages, but that's kind of the way it works. Depending on whether you select "Expert" mode (which lets you make all of the adjustments yourself) or one of the preset modes ("Optimal", "Aggressive", or "Turbo"), you can either change the timings yourself or they will be changed for you automatically whenever you change the memory bus speed. Some of the latencies look pretty high to me, but there's a lot of room for experimentation. I usually just stick with standard speeds like 200MHz and standard timings myself, but if you want to overclock your memory, you do have that option. I don't know if you could run your PC2100 at exactly 145MHz, but you could probably choose a speed pretty close to that.
Putting a fan on the NorthBridge wouldn't hurt, but it doesn't seem to get very hot to me--just warm to the touch. Unfortunately, the CPU socket is perpendicular to the NorthBridge, so the fins on my CPU cooler are parallel to it (if you follow me), so the NorthBridge doesn't get much air flow directly from the CPU cooler. On the positive side, the heatsink on the NorthBridge is fairly large for a chipset heatsink. I haven't pulled mine off yet, but if you wanted to, you could remove the heatsink (it attaches to the motherboard with two spring-loaded pins) and use your own thermal compound (Arctic Silver 5 comes to mind) to make you feel more confident about your cooling solution. Or, you could replace the heatsink with a really good chipset cooler. But if you're really pinching pennies, it's probably not worth the expense. On the other hand, if you run your memory at 400MHz (as I recall, I think that's an option) and your processor at a 250MHz bus speed (I know for sure that's an option), then maybe you do need more cooling.
87pathfinder, I just read your post. I'm glad that you concur with me about the Shuttle AN35(N)-Ultra, and I'm glad that you have two CPUs that overclock well. I'm just wondering, have you considered putting your unlocked Athlon XP 2000+ in your Shuttle "toaster box"? You didn't say how hot it gets. What I'm thinking is that you could max out your Mobile Athlon in your desktop computer and get better performance than you could with your Athlon XP 2000+, and maybe you could optimize the heat/performance ratio of your portable Shuttle system if you put the Athlon XP 2000+ in it. It's just a thought. Also, do you think that using a 166MHz bus speed instead of a 200MHz bus speed has any advantages, heat-related or otherwise? I haven't tested that myself, and I'm curious what you have found out.