I built a fairly similar computer a few months back:
- Intel Core i5-750
- Asus P7P55D-Pro motherboard
- 2gb ram (single stick for now, waiting for 4gb sticks)
- ASUS Nvidia 8400 GS "Silent" w/512 gb ram (waiting for cheaper vid card)
- 1.5 Tb HDD
- 550w Ultra power supply
- NMedia HTPC case (had major QC issues with this case)
I'm running Win 7 64-bit Home Premium, and Ubuntu 64-bit 9.10 on it, having some issues with Ubuntu not being able to run a custom resolution to compensate for overscan on my TV, other than that it's a great system.
Max memory usage so far has been roughly 1.2-1.3 Gb and that's with my average use (copying, converting videos, music, running game emulators, etc.). I did a mild overclock on the system considering I'm using the stock cooling fan, and it's running 100% stable at about 7% OC, just under 3.0 Ghz if I remember correctly. Did quite well in Everest benchmarks... I'm plenty happy with the i5-750 for a budget processor!