Hmm...okay. First you would want to upgrade mobo/processor. 64 bit is not needed ow but in 2 years windows 64 bit os called longhourn will come out and this will require a 64 bit processor so the 64 bit processo would be future-proof. As for ur vid card, a 5200 would be able to play current games but in low settings....i would believe. Then after the processor/mobo, you want to upgrade the ram. If your a heavy gamer, its nice to have 1 gig but if u cant afford get 512 now and then 512 later. Get some pc3200 cas2 or cas 2.5 ram. You don;t want cas 3 ram as it isnt that fast. Also get a name brand stick of ram like...kingston hyperX, OCZ,Mushkin or corsair or geila. Your hd would come next and i would go for a sata hd. SOmething like WD,seagate, or maxtor would be nice in sata form. As for how much it costs, well go on www.newegg.com
and check. cheers.