Advice on $800 gaming build

APPROXIMATE PURCHASE DATE: this month

BUDGET RANGE: ~800 before rebates, but I'm flexible

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: gaming, surfing the net with many windows open at once, listening to music

PARTS NOT REQUIRED: Keyboard, mouse, monitor, speakers

PREFERRED WEBSITE(S) FOR PARTS: newegg

COUNTRY OF ORIGIN: United States

PARTS PREFERENCES: No preference really

OVERCLOCKING: Yes, after more research

SLI OR CROSSFIRE: Maybe (would like to leave possibility open)

MONITOR RESOLUTION: 1280x1024. I don't know if I want to buy another monitor now or any time too soon, maybe in 6 months to a year.

ADDITIONAL COMMENTS: I want a system that can play current gen games at at least 1280 res. Thing is, if I really build the system I want I'd probably end up with an i5 750, two WD black 640gb hds running in raid 0, a 5850 gpu, ect (this may be not ideal, but seems like a good system to me based on my research). But then, I'd also need to buy a new monitor to take advantage of the GPU and the whole thing would cost me $1200-1400. I can spend that I guess, but I don't know if that's really worth it to me. I think for the money, I'd be happier with a phenom x3 build and a 4850 or 4870 card with my current monitor. I feel like I can buy that for under $800. Is it worth the money to go straight to an i5 system, or would i be ok with the x3 for now? From what I've seen in benchmarks tests, if I OC it i'll be able to play most anything I want with decent graphics settings. I want to be able to upgrade in the future, like a year or so.. Something like buying a new socket am3 cpu, additional ram, new GPU and new monitor. Is this feasible?

In addition, I haven't been able to really understand how to select a mobo and ram, even with all the research I have been doing. Can somebody recommend a basic build for either of these? I'd like to incorporate the:

COOLER MASTER RC-690-KKN1-GP Black SECC/ ABS ATX Mid Tower Computer Case - Retail +
Western Digital Caviar Black WD6401AALS 640GB 7200 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive

combo for $100, long as the case is good for my purposes. I know I am asking for a lot, but I really am having trouble deciding on which is a better idea.