Is there any particular reason why you have chosen Socket 775? This is an old socket now so components are on the rise in price. For roughly the same price, you can get one of the newer sockets with an iSeries processor (i3, i5, i7).
For gaming, I'd recommend either an i5 or a well overclocked i3. Both use Socket 1156. It's also DDR3 Memory which is much faster than the DDR2.
I'd also recommend a better Graphics Card if you're planning on playing newer games. The nVidia GTX 460 is a very good price for the performance. ATI have also just released the HD6xxx series cards which are also at very good prices.
Really, you want to be going above 4GB memory. 6GB would be fine but it means you would need to replace some of it in the future if you wanted to upgrade to more. 8GB would be more than enough if it's within your budget, but 4GB would be ok for a while too.
For the Motherboard, I tend to prefer to Asus Intel chipset boards. They're very good quality and decent price. the Asus P7P55D is a good board.
Let us know what you budget is and we will spec you up a decent machine for your budget.