You may want to have a look at www.newegg.com
, even if you decline to do business with them. Their "power search" feature is convenient. It would allow you to choose (say) Socket 1156 and a number of SATA3 ports. (Make sure to avoid an "open box" deal, though. It's discounted, but all that Newegg guarantees will be in the box is the board. It may lack the I/O shield.)
The choices are complex. Socket 1156 motherboards are cheaper on average than Socket 1136 ones, but their price ranges overlap. Socket 1136 supports triple channel RAM, so most people buy that, at a higher price than the dual channel on Socket 1156. The I7 CPUs (including the Socket 1156 ones) have 4 cores, and support hyperthreading, for 8 logical CPUs. The I5 750 has quad cores, but no hyperthreading. (That's usually unimportant for games, but maybe significant for multi-threaded applications.)
At Newegg, the I5 750 sells for $185. This motherboard: Newegg.com - ASUS P7P55D-E LX LGA 1156 Intel P55 SATA 6Gb/s USB 3.0 ATX Intel Motherboard
is $125. (Two SATA3 ports, two USB3.) 4GB (2 X 1GB) of DDR3 2000: Newegg.com - CORSAIR XMS 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 2000 (PC3 16000) Desktop Memory Model CMX4GX3M2A2000C9
is $120. (There is cheaper, and probably as serviceable, RAM out there, but I've had good luck with Corsair.) The tab is already at $430.
Of course, if your budget will stretch further, an I7 930 is $290. A Socket 1366 mainboard with USB3 and SATA 3 is $200: Newegg.com - ASUS Sabertooth X58 LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 Intel Motherboard
A 6GB (3 X 2 GB) kit of DDR3 2000 is $175: Newegg.com - CORSAIR XMS 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 2000 (PC3 16000) Desktop Memory Model CMX6GX3M3A2000C9
You'd need a 64 bit version of Windows 7, with 6GB of RAM. The total for this is $665, plus shipping. This is a more powerful system that the I5 one, but at more than 1.5X the core cost.
If you wish to overclock, invest (<$100) in a third-party CPU cooler. The best (air) ones tend to be large, so make sure that you buy a case that'll take one. If you intend to buy a high-end graphics card (nVidia GTX480, ATI Radeon HD5870), that'll also need a case that can take long cards. Get a PSU (power supply) with lots of margin (driven by the graphics card).