Heres my experience from having swapped between a msi hd4770(reference)->his hd4850 512mb(non-ref)->galaxy gts250 512mb(non-ref)->msi hd4670->hopefully hd 4770 next.
I'm in a similar position as the op, picking out a new htpc card and want the 4770 but can't find it in stock. in the mean time, I've tested the cards listed above and here is my research and real world observations from a HTPC standpoint.
1. GTS250 is not a 9800gtx+. More efficient PCB design, more mature 55nm process, lower gpu voltage, down clocking when idle = better lower idle consumption than the reference 4830, 4850, and 4770. Cons - 2d downclock isn't implemented on some non-reference designs (like the galaxy i bought). not as efficient at load as the 4770.
2. his 4850 - I was hoping this non reference design would be a good htpc card (since it was shorter and had a less complex power circuitry) but it does not downclock at idle. What a disappointment. Seems to be more efficient than the reference design at load however. An interesting card is the gigabyte 4850 multicore. Software adjustable gpu/mem voltage and it downclocks at idle. With some tweaking, that could be a very powerful and efficient htpc card (but i dont have the cash to test that theory).
3. MSI 4670 (.95v/1.15v) idle/load - In the end, I went back to my old card which is the reference 4670. 93 watts system power consumption at idle vs. 125+ watts (i forgot the exact numbers) on the galaxy gts250 and his 4850. ~135 watts playing left 4 dead on the 4670 vs 190+ (again, dont remember exact) on the 4850, even more on the gts 250.
My recommendations for HTPC.
1. Buy a recently manufactured card regardless of model. Mature manufacturing process = better efficiency, lower voltages.
2. Buy reference or make sure the non-reference downclocks at idle.
3. Buy a card where you can tweak the voltage, fan, and clock settings. Most cards offer this through bios adjustments. Some special cards with volterra or gamer hud regulators (like the gigabyte 4850 multicore) can do this in software which makes tweaking them even easier. Beware of non-reference cards that can't be adjusted. Running at lower volts = huge power/heat savings (especially at load). I speculate the majority of the increase in power consumption between the 4830vs4850 is the higher gpu and memory voltage needed to run at 4850 speeds.
4. Keep your card/system as cool as possible. All else equal, a cooler video card/system will be more power efficient.
5. GDDR3 > GDDR5 unless you need the bandwidth. No card with GDDR5 can downclock the memory at idle (as far as I know). From a HTPC standpoint, a 4870 runs 30+watts hotter at idle because the gddr5 doesn't downclock. You can manually force it to downclock but it will result in a flicker everytime the clock is changed.
6. Don't buy more powerful than you need. For example: The most graphically intense game I currently play is left 4 dead which runs at max settings 1920x1200 on my 4670. Buying a 4870 would increase my frames even further but I won't notice the difference in real world gameplay. But it will probably increase my system power consumption at load by 100+ watts.
What card I recommend:
Sapphire 4830 - From what I know, this card automatically downclocks at idle. The voltage can be adjusted as low as ~.9v. Improved pcb design. More mature 55nm process. I speculate you can lower the 3d load voltages and obtain efficiency comparable to that of a 4670. cons - unnecessarily powerful if you dont need the speed.
hd 4770 - with gddr5 and 40nm, best efficiency period. As 40nm matures, these cards will become even more efficient. cons- high idle consumption due to gddr5 that cant downclock at idle (and leaky 1st generation 40nm?). poor availability. incompatible with popular passive coolers like the s1/s2. Not as cost effective after recent ati price cuts.
*power consumption measured by my kill-a-watt on my gigabyte ep45-ud3p, intel q9550, 2x2gb memory, x-fi xtrememusic, avermedia tv tuner, lite-on dvd burner, 2x1tb WD GP, 1x WD 640gb, 1x 30gb ocz vertext ssd.
EDIT: One of the best power consumption/video card articles I've read and where I learned a lot of what I've listed above.
http://ht4u.net/reviews/2009/p..._consumption_graphics/