the i72600 also gives you four more cores over the i52500. for 10 on one box, i would highly reccomend the 2600i7, if your budget allows the i7 3930k would do even better (12cores vs 8cores) the most clients i ahve "playably" launch on the 2600k is 13, and i have gotten 18 playable on the 3930. both using multiple gfx cards. the gtx580 3gb would be your best viable option for using a single card for around 10 clients. Going with as much ram as you can get, and some ssd's in raid will also tremendously help, and as far as the PSU goes, i wouldn't even think of anything under 750 watts. Once you get things loaded up, and even with a mild overclock, your gonna want the extra headroom. Single rail powersupplies are better as far as ease of setup (not watching what rails your loading during build) they tend to cost a little bit more, but are worth it in the long run IMO.