Listen, if you honestly want to balance timings and prices, just get this then:
Kingston 1GB PC3200 400MHz 2-64x64 HyperX DDR Memory - $170
333MHz (PC2700) 2-2-2-5-1 (CAS Latency 2)
370MHz (PC3000) 2-2-2-6-1 (CAS Latency 2)
400MHz (PC3200) 2-3-2-6-1 (CAS Latency 2)
434MHz (PC3500) 2-3-3-7-1 (CAS Latency 2)
466MHz (PC3700) 3-4-4-8-1 (CAS Latency 3)
500MHz (PC4000) 3-4-4-8-1 (CAS Latency 3)
Like i said before I'd go for cheap 1 gig but if you feel like timings are somewhat important, and you don't want to pay an arm and a leg, this is pretty much as good as it gets.
Sell the old stick to finance this purchase. So if you sell the old stick for say $50, this upgrade will only cost you $120.
Finally, if you dont want the hassle of selling your old stick. You can always pick up an extra 256 + 512mb of ram, which together with your old ram will give you 1 gig.
Obviously ram is going to provide MUCH less benefit after first 512mb compared to a videocard upgrade in the latest games. So depending on how much you want to spend:
1) keep the old stick, pick up an extra stick of 256 now for a total of 512, and 512 later
2) sell 256 you have and get 512 now and 512 later
3) sell 256 and get 2x512 now.
4) keep the old stick, pick up extra 256 + 512 for a total of 1 gig now