im going to give you advice rather than exact parts.....
os- win7 home premium 64 bit
ram- look for
-ddr3 1333
-4 GB (2x2gb)
-a reputable brand (kingston, crucial, corsair, g skill, patriot)
-priced at $100 +/- 5
-lower cas latency is better
motherboard- look for:
-at least 4 star avg review (newegg.com)
-socket AM3
-4 ram slots
-2 PCIe slots so that you can upgrade to crossfire later (if you dont think you will do this, then 1 is fine). make sure none are x4 speed, x8 is fine; it should be x16 when only using one card and x8/x8 when your using 2, (or better, like x16/x16)
-good brands in general: gigabyte, asus, msi
-bad brands in general: jetway, biostar, elite group
-dont get a micro atx (limited upgradability)
-the 890GX northbridge chipset would be perfect for you
hard drive:
-western digital is best, seagate has hit or miss reliability, same for samsung
-buy whatever capacity fits your needs and budget
-black edition drives will be fastest
-7200 rpm, high cache
-ssd and 10000rpm drives are out of your budget
optical disc drive:
-can be had for $20-25
-make sure its sata, not ide
Case:
-spend at least $50, get something nice, i regretted cheaping out on my case
-best bet: antec 300/antec 300 illusion
-shipping is expensive for cases, look for ones with free shipping
power supply:
-again, spend at least 50, bad idea to cheap out on psu
-at least 2 pci-e connectors, best to have at least one 8 pin or a 6+2 pin
-as low as 450 would be fine for a system like yours but for upgradability (crossfire) get a 600 watt, google for power supply calculators and always give some margin
-modular is nice, but far from a neccessity
-high efficiency (80 plus, etc) means less heat and thus less noise, but remember that a 500 watt psu outputs 500watts...doesnt matter what efficiency it has,
cpu-
overclocking? then phenom 2 X4 955
not overclocking then phenom 2 X4 945
graphics card
-get whatever card you can afford after you've choosen all the other components
-minimum: ati radeon 5770, but get a 5830, 5850, or 5870 if cost allows
-all brands are usually good, just look at reviews to choose a specific card. i like XFX personally. powercolor is kinda cheap
other tips:
-be sure to look for combo offers and other special deals.. you can end up getting alot more than what you pay for
-always read the one star reviews
-ask yourself what your upgrade path will be, if you plan on upgrading before building a new system
this means more money is spent on the motherboard and psu
-shipping can add up, consider this in the budget