Wow, I just checked when I got my computer: November 2009! Almost 4 years ago!
$280 - Core i7 920 overclocked to 3.2ghz
$310 - 12GB RAM (so much more expensive back then)
$270 - Gigabyte EX58 UD5
$200 - Coolermaster ATCS 840 -- I LOVE this huge roomy, convenient case.
So about $1000 (I had a video card at the time).
Later, my friend showed me a good deal on an nVidia GTX470 for like $250 or something.
Oh, and of course I've bought SSDs -- some of the most important parts of the system.
Later I upgraded my RAM to 24GB for $130! I actually do use this for large-scale computations.
Overall, I've never felt any lacking despite not upgrading the mobo/CPU for 4 years, and the video card for about 2-3.
Advice: Buy what Things suggested above. You don't need a super fast CPU. An i3 will do just fine for most things. Look for CPUs that you can overclock and you'll be able to get extra speed without dropping the extra cash.
My personal advice: MAKE SURE to get an SSD for your system drive and games (e.g. Crucial M500 120GB for $110). You'll hear some people poo-poo the SSD as unnecessary, but they are wrong. They usually have never used one and think that a lot of bulk space for media they never watch is worth more than ultra-fast disk access for what you really need -- even with outrageous amounts of RAM to compensate. Overall, your computer will be super snappy, and may run better than a computer like mine that I spent an assload for years ago, but never feels sluggish.
You'll have all your old hard drives anyway, so you can just move them to your new system and use your new SSD as your system drive. Watch your system boot in 5 seconds after POST; laugh as your games load up super fast while your friends' crappy machines are loading the texture data and maps from their shitty platter-drives; never have to defrag your drive because it works faster than even Velociraptor RAID arrays without any moving part.