wouldn't be the 64, and I reckon you won't find an Athlon XP3200 system in a hurry nowadays unless you go to EBay.
You can get Intel celeron systems cheaply which will play Halo, but one can hardly give any sensible advice without knowing what "cheap" is. After all, you can get refurbished PC's for £150, but would you buy one? and why would you buy a PC just to suit that game when chances are you'll end up getting another game at some point which will need a PC more advanced than the one you bought to use Halo on! That's why the old adage of "get the biggest, fastest machine you can afford" stands true, because in reality you will end up with something that will have a longer "lifetime" than the el cheapo 5 year old system you bought for £100 from the guy beside the fireplace in the pub.