That wouldn't run Crysis at all well. The 2600XT (and for that matter, the Geforce 8600) are midrange cards with mediocre performance. What is your budget? If you want to get a prebuilt, and do any games, the Dell XPS 630 is a great choice for a budget. Of course, the best value will always come from building it yourself.
2.0 is no big deal at all, I don't know about it voiding the warranty, but that is a good base system and by popping even a 9600GT in there you would get a decent Crysis performer, definitely able to do medium.
2600xt cannot run Crysis high settings at that resolution at playable frame rates, not by a long shot.
Here's a frame rate chart of different cards. Note that they're average frame rates, and on heavy scenes, it will dip below that. Also, it's at medium settings, high will decrease fps further. The conventional wisdom is you'd need at least a 8800gt (around $120) to get playable frame rates in Crysis (35+) on high settings.