After some long research; it seems that these 2 cards are the best that you can get to upgrade a slimline HTPC. I understand niether card are top line for gaming, but will have to do as selections are limited. My system has enough to run these cards and my OS is Win 7
The galaxy 9600 gt has~ DDR3 512mb, 256 bit, 64 stream and supports DX 10, 4.0 shader and Open GL 2.1
The Sapphire HD 4650~DDR2 512mb, 128 bit 320 stream and supports DX 10.1, 4.1 shader and OpenGL 3.2
The 9600gt is a couple of tiers higher on the performance chart but it seems the gaming tech is newer on the HD 4650 will this matter much?
The 9600 GT low profile low power uses less than 75w at max so your PSU is fine. I got mine yesterday and installed in a custom built slim case PC. Total power consumption is around 150w at the socket. Your PC will use slightly more but consider how inefficient my PSU is, your max should be under 200w at the socket. Your 240w system load equals to about 300w at socket.
Athlon 64 X2 4000+ @2.9GHz
WD 750GB HDD 7200rpm
No optical drive
350W PSU (crap that came with the case ~60% efficiency)
I got mine from TigerDirect for $109. There is a Galaxy $30 rebate and 12.5% Bing cash back so the final price is around $65.
I currently own both of these cards. I use them in my Gateway sx2800-01 which only runs a 220W power supply. I have played Far Cry 2, Bioshock, and Fallout 3 with High to Very High settings. The HD4650 struggles a bit with Fallout 3's massive amounts of shading. Bioshock is not a problem on the HD4650, even on the highest settings. Far Cry 2 fares a little better than Fallout 3 on the 4650, but still suffers from having the "open world" shadow problem to deal with.
I just purchased the Galaxy 9600GT low profile low power card two days ago. It was a pain to install, but it's doable. There is a noticeable difference in my pc's ability to play Far Cry 2 and Fallout 3 now. The 9600 handles those games much better than the 4650. However, I play it with my PC's cover off to increase airflow. I played Far Cry 2 for 3 hours straight, on Very High settings, and the GPU temperature never got above 78 degrees Celsius.
What's funny is that, on my SX2800-01, the HD4650 actually runs about as hot, sometimes hotter than the Galaxy 9600GT. The HD4650 could get up around 85 degrees Celsius.
I do want to get a new case and PSU to better handle everything, but for the time being that is what I've found between the two cards. I still like the HD4650 and I'll keep it as a backup for the 9600GT. You may have found all that out for yourself by now, but maybe that helped somewhat.