I won't go so far as to suggest particular items as you may want to mix and match in line with your budget and availability.
But as general guideline I'd recommend a good dual core CPU over a quad core. XP OS and a mid range nvidia GPU (driver stability advantage over ATI) and don't be stingy on the RAM. 4 gig is more then sufficient but get the fastest your budget allows for. Get a good mid range motherboard but don't opt for an onboard sound card unless you really must.