For everyone who always complain of around $2k requirements to build a computer capable of running Crysis well, Crytek put together a computer for around $900 and that'll run Crysis well.
Note that it doesn't include mouse/keyboard/monitor but if you have a computer period, you should already have those.
http://kotaku.com/345278/crytek-specs-out-an-affordable-crysis-pc
http://www.incrysis.com/forums/viewtopic.php?id=18017