i run a 3.2G P4, and that's more than enough for CS. ppl have had a lot of trouble with running dual core CPU's, so be prepared to fiddle if u get one.
go for PCI xpress for video card. Mine is 8xAGP (ATI X800XT) and cost a fortune last yr, and barely cuts it now. be prepared to spend over $500
hdd, go SATA for sure
RAM - go 2Gb. 1 Gb is barely enough now.
screen - 8ms 19" LCD is good (but I still have an old CRT desk bender)
case? lots of fans and big power supply (> 450W) is necessary cos vid cards and CPU really suck watts these days
but doubt - u do fine in CS, and are still a joy to ghost