Just as a note here, memory doesn't have a whole hell of a lot to do with framerates. Core speed is what you're looking for here, so the trick is to find you a decently priced card with a decent amount of RAM. From the fact that you own an X300SE, I know that you've got a PCI-e slot, so here are some potential choices (although, as you're out of the US, you will probably need to find a different retailer) that seem appropriate for you.
DX10 (if you intend on eventually going to/already have Vista):
8600 GT:
http://www.newegg.com/Product/Produc...82E16814150229
8600 GTS:
http://www.newegg.com/Product/Produc...82E16814150228
ATi's midrange DX10 offerings don't offer adequate performance, which is why I have not included them.
DX9 (if you have no intention of ever going to Vista):
X1950 Pro:
http://www.newegg.com/Product/Produc...82E16814121031
7900 GS:
http://www.newegg.com/Product/Produc...82E16814133186