Ok well either the Corsair PC3000 or PC2700 XMS would be nice. There is no 1 DIMM 512MB PC3000 XMS so you would have to get 2 256MB sticks, which would cost $244US. (122*2) One 512MB stick of PC2700 XMS costs $192 on pricewatch.com.
OEM versions of the AXP2100+ are ~$230US
Retail version of the AXP2100+ are ~$250US
OEM version of AXP2000+ are ~$175US
Retail version of AXP2000+ are ~$210US with retail HSF if you're not going to oc.