Which Corsair XMS3 6GB kit were you looking at? there's 6 different ones with different frequencies and timings. The cheapest ones aren't all that great, some of the mid-priced ones are okay. My favorite pick on RAM from last week just went up in price it was a DDR3 1600 6GB Kit from OCZ that had timings of 7-7-7-24. Goes for $146 now though.
If you can wait a week or 2, you might want to wait for the ATI 5870's to come out, even if you decide to stick with the 4870, it should be cheaper after the 5870 is available.
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode)
Certified power supplies are recommended. Refer to http://ati.amd.com/certifiedPSU for a list of Certified products.
Are you reusing that Samsung F1 from a previous system? I can't even find that model for sale anywhere. If your budget allows, you might want to consider upgrading to a WD Caviar Black. Anything in the 640GB - 1TB range will give you much better application performance, and you can still keep the SpinPoint F1 as a storage drive if it's being reused.