A great price differ will be what CPU you want to use. For your budget I would have Bought a s775 Motherboard, p5q3 as I bought myself, which supports both Quad, Duo and Extreme CPUs from Intel. I recommend Corsair 2x2Ghz DDR3 Ram DIMM for that.
Graphiccards is a big question, but the GTX Series isnät very Expensive for all cards but great Performance. GTX260 to recommend, and depending on how many GPU's you want to use, you will need different amout of Watts in your PSU. 600W is allright for allroud using if you ask me.
If you want a great improvement, you change Motherboard to s1366( I think )and install i7 processor instead, which will cause you computer to last for longer before you need another upgrade, but it's a question about money.
i have been a fan of the AsusTek company and prefer their parts infront of other manufacterers.
HDD should be a WD Black, 500Gb-1.5Tb depending on your need of memory.
Optical reader is, in my opinion, just something cheap. not much more then $30 should be spent on this.
If you feel you aint in need of some expensive parts, reduce their performance and price and upgrade what you feel you going to need