Your friends don't know much about computers :roll:

Are you looking to put it together yourself, or buy one premade? Some of the better multiboxing-on-one-pc rigs on this site are going to be good for Crysis, with the exceptions that you will want to run SLI video cards as stated above.

Something along the lines of:
Higher-end Core 2 Duo / Core 2 Quad processor with some serious cooling so you can overclock the crap out of it.
3-4gigs DDR2 800 or faster ram
2x hard drives in a RAID0 config, preferibly 10,000 RPM drives
2x Nvidia 8800GT video cards in SLI
A monitor that uses a DVI or HDMI connector. Most everything decent does these days, and it makes a noticeable difference in picture clarity.

If you buy a pre-built system from a major manufacturer, most of them come with a bunch of pre-installed BS software that will slow the system down by as much as 50% (No I am not kidding), so if you take that route, you may want to see if there is a way to get a clean install of your OS, (OS, drivers, no crap), or at least uninstall the crap that comes with the machine.