I originally spent about $1800 on my PC in late 2007 (which I have continued to upgrade rather than outright replace since then). It was just good enough to play the game [Crysis 1] on Very High at about 25-30 fps continuously, hitting maybe as low as 20 fps in certain spots on a few maps.
I could go on about my rig but it's not anything spectacular now (Q6600 @ 3.0, 4gig DDR2 ram Win7 etc). Last changed I made were my 8800GTX -> GTX280 and putting in high performance SSD for my OS and a few choice games and a 1080p 21.5" monitor. I expect i'll run Crysis 2 fairly well, though if you're looking to build a rig with the kind of money I spent then on today's technology, you could get something better than I have, as is the nature of PC gaming.
It really depends on what you spend; Establish a budget ceiling to start off with. I build my own pcs from pieces as that's the best way to get the bang for your buck. I used newegg.com
I would say you would need between 1200-1500 for a good rig that could run the game well. Obviously you could spend more than that and get an even better rig, but it also depends on if you're able to build it yourself and what kind of brands you're going to buy. It's fairly involved, but if you're up for the challange I would begin doing some research and take a month to gather together information before you buy any parts. A really good PC will last you for years if you do it right.
I'm sure there are other folks on here that are a bit more versed in the absolute latest stuff. I always take time to read about the latest technology whenever i'm in my upgrade cycle before I purchase anything.
Haha, I remember upgrading my computer for Wolfenstein 3D and Doom good times, good years.