Help me buy 1/2 a computer!

I've been a TH lurker for a while now...let me finally say, you guys are incredible, and thanks for the ongoing tech lessons.

So...I've been upgrading my xp dell somewhat intelligently for 5 years. But I'm ready to make the leap to a new mobo/processor.

This is my first solo build...I'd like to end up with a mid-high end gaming machine of the i5 processor variety. Perhaps with a bit of overclocking on the graphics and cpu.  

Components I have (and want to use):
2 1.5 GB SATA Drives
Radeon 4850 HD

Components that might be useful...or junk?
4 1GB DDR2's, 667 gHz

Components I'm semi-settled on:
Win 7 64-bit OEM
Intel i5 - 750ish

TBD:
MOBO
Ram (if above is unsalvageable)
Case (looks mean nothing, ability to keep overclocked components cool = everything)
Power Supply (600 ish?)

I can probably afford to spend $500-$800 here...and I'd consider i7, etc, but I don't want to leave my current components completely in the dust here. Looking for balance.

I'm a mid-level gamer...I don't see myself growing into dual graphics cards. Just want some good frame rates, ability to cut some home movies...and a little bit of upgradability.

Greatly appreciate the advice!