You can game across 3 monitors with Nvidia too just for the record, it just has a different name. 'Nvidia Surround'. Also, overclocking is increasing the speed of your hardware but generating more heat, so if you have good cooling it's recommended. AMD cards are probably a better choice in GPU's for Eyefinity as you tend to get more graphics memory and that's important for multi monitor gaming. Here is what youre going to want to aim for:
Intel Core i5-2500k
2x4GB G.Skill RipjawsX 1600Mhz
SAMSUNG Spinpoint F3 1TB
XFX Core Edition PRO850W
COOLER MASTER HAF 922
LG CD/DVD Burner
SAPPHIRE Radeon HD 6950 2GB (One of these for medium-high settings, probably 2 for ultra high)
3 x ASUS VW246H Glossy Black 24"
Windows 7 HP 64 Bit
Before shipping and rebates and including both cards and all 3 monitors that comes just over $1900 on Newegg. Maybe just make do with one GPU for a while so you have some cash for a decent mouse and keyboard and whatever else you need. (Speakers?)
You may need a display port adapter too as one of the monitors in Eyefinity has to be connected by displayport as far as i know.
Anyway, this build would be x8/x8 Crossfire ready (Crossfire is recommended for decent framerates if you want very high settings in Eyefinity) and good for overclocking. You wont need to do this until the CPU starts to show it's age, that gives you plenty of time to learn overclocking.
Lastly, i would highly recommend reading plenty of guides on how to build your own PC before actually doing it. It isn't as hard as it might seem when you know what youre doing though so don't be intimidated.