Need advice: budget data crunching PC/upgradeable to moderate gaming

APPROXIMATE PURCHASE DATE: basic configuration = now; can add non-essential parts later

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Heavy data analysis, multitasking, Gaming.

PARTS NOT REQUIRED: anything outside the case

PREFERRED WEBSITE(S) FOR PARTS: newegg.com is fine (deals from Fry's are good too)

PARTS PREFERENCES: none

OVERCLOCKING: Yes, if it's relatively straightforward SLI OR CROSSFIRE: Eventually

MONITOR RESOLUTION: 1920X1080 dual monitor setup

ADDITIONAL COMMENTS:

Budget for initial build = aiming for under $600. Additional $500 for upgrades later in the year.

Basically, I'm building a PC for home that I can analyze data from work with (I'm doing neuro research and have huge data sets that take forever to open, much less manipulate in Excel or LabView, on my dual-core XPS 400) as well as get decent performance from games with. Initially, it's most important that the PC is excellent at processing non-graphical data, so I wouldn't mind adding (not replacing) parts that would improve its performance as a gaming PC that would exceed the price range I've specified later in the year--but the PC has to be complete enough to do work ASAP.

In total I'm hoping to get recommendations for the initial build, and also suggestions for additional components that would be good to add in the next 6 months to make the PC more suitable for gaming.

I'm green, so I'd like have a system that doesn't suck electricity gratuitously, and has good solutions for cutting power use when idle (eg. I think I've seen some Asrock boards that supposedly were able to nearly shut the PC down while allowing Windows to become functional within a few seconds when woken up).

Thanks!