im guessing youre talking about the AMD phenom ii x4 965 because its in the same price range as the i3-2100. If you overclock the phenom to 3.7 ghz, you average about 6 FPS less than the i3-2100 for starcraft 2 on maximum settings at 1920 x 1080. http://www.tomshardware.com/reviews/gaming-fx-pentium-a...
If you just play SC2, you probably want to get the i3. I personally like the i3-2120 because its clocked 200 mhz higher and only $3 more. Other games such as battlefield 3 multiplayer, diablo 3, deus ex, witcher 2, MW3 and others use more than 2 cores. So if you plan to play games that use more than 2 cores, you might want to sacrifice a little performance in SC2 for gains in other games.
Granted starcraft may be optimized for intel CPU's but it is still a fairly CPU intensive game meaning that it will use %80 of the i3's effort. While a 4X black edition phenom of any sort will have no issues hitting 4Ghz (matching the locked i3's performance) and the extra 2 cores will make the system much more responsive and capable while gaming (alt+tab, running other programs while gaming, downloading so on and so forth)
Would H61 mobo be fine with the i3 for my purposes? I likely will not upgrade this system. Probably go 4GB ram. Can anyone suggest a motherboard? Can anyone suggest a graphics card similar in price to the i3? I'm not looking to run SC2 on ultra.
if you aren't looking to upgrade your system then the AMD system might be better for the long run as 4 cores will be quite useful in some other things. The difference between AMD OC and i3 isn't much in sc2 but in other thing the quadcore really helps the AMD system push ahead.