Looking for input on my current build

So i have been blitzing toms hardware and other sites tryin to understand how todays hardware works for a new pc im tryin to build, last pc i built was 5 years ago so its been a while ;(. Main use is for gaming i guess and misc schoolwork sometimes using autocad etc.

So far this is what im leaning towards:

CPU
Intel Core2 Duo E8400 3.00GHz 6MB/1333FSB S775 CPU, Retail [ BX80570E8400 ]
Arctic Cooling Freezer7 Pro S775 Cooler

MB
Asus P5Q-E Intel P45 S775 ATX Mainboard (Audio/Dual Gigabit LAN) [ P5Q-E ]

RAM
OCZ Reaper 4GB DC PC2-8500 DDR2 Memory Kit *EPP* [ OCZ2RPR10664GK ]

HD:
WD SATAII 250Gb 7200RPM 16Mb Hard Drive, 3Yr [ WD2500AAKS ]

VID CARD
Sapphire Radeon HD4890 1GB PCIe Video Card w/2xDVI/TV Out, Retail
OR
Sapphire Radeon HD4850 1GB PCIe Video Card w/HD/TV-Out/2xDVI, Retail

CASE
Thermaltake Armor + MX Mid Tower Case, Black [ VH8000BWS ]

PC Power & Cooling S61EPS 610W Silencer Series, Black [ S61EPS ]


Now my reasoning for the MB and CPU is that info i have gathered says its easy to overclock to 3.6ish and it seems not many things take advantage of quad cores atm so faster dual core would be better.
RAM im not to sure about aside from info that PC8500 RAM works best with the MB so i just looked for some RAM from a good manufacturer.
I dont need a big HD since i have two alrdy just something fast which is what i chose i think.

Vid card is hardest decision atm, im not sure if i should go with a single 4890, single 4850 or xfire 4850s right away. Like i was thinking getting one 4890 now would be plenty of oomph, and then i could add another later if needed. Or are xfired 4850s a large enough preformance increase over a single 4890?

For case i liked the thermaltake armor series look but went with mid tower instead of full. Are mid towers still roomy enough to fit everything and have room for new upgrades and cooling or is it better to just stay with a full? PS is one that was crossfire ready so i can be ready to do that whenever i want.

. I guess my main goal is to build a fairly powerful rig that is upgradeable in the future so havin xfire and ability to go upto quadcore seems good.

Any thoughts, tips or recommendations about this setup would be greatly appreciated.

Cheers