Here is another option, one that would help you avoid compatibility problems with your current stick...buy a 2 x 512 kit of PC3200.

A 2 x 512MB set of value ram from Corsair or similar can be had from the egg or other similar online retailers for pretty cheap these days. Here is an example:

Patriot 2 x512 set of PC3200 (2.5-3-3-7 timings) for about $75, shipped
http://www.newegg.com/Product/Produc...82E16820220037

A simple search at the egg will get you a bunch of similar ram as the one linked above. If you stick with a name brand (Corsair, Mushkin, Kingston, OCZ and even Patriot) then you should have not problems with quality, plus many of these will have lifetime warranties, should a problem subsequently occur.

To me the advantage of buying a 2 x 512 kit are as follows:

1) No concerns with compatibility with your current 256 stick. The current value of a 256MB PC2700 stick is likely to be in the $15-$25 USD range and not worth (in my mind) the aggravation of it possibly not playing nice with a new single 512MB stick. Just park your 256 stick for emergencies, like an RMA need, or sell it (for the price of a burger, fries and soda) or give it away.

2) PC3200 will downclock to PC2700 speeds and usually will do so at tighter timings so while the rated timings at PC3200 speeds might be 2.5-3-3-7 (like the Patriot above), at DDR333 speeds those timings could perhaps be closer to 2.5-3-3-7 (just kidding...don't know what the SPD timings will actually be at DDR333 speeds due to the various chips used, but usually they are tighter than at DDR400 speeds, but will vary from kit to kit).

3) Resale value of a 2 x 512 kit will be better and in higher demand than trying to sell off a single 512MB stick, should you later upgrade your computer and go with either 2 x 1GB set of perhaps a DDR2 rig. The difference to you in net cost from buying a single 512 and then selling it vs buying a 2 x 512 kit and later selling it will not be that great and you would have avoided the compatibility issues mentioned in (1) above and have enjoyed the performance benefits of a full gig during the time you used it.

4) Depending on your next upgrade...you might even be able to use it, should you go with a Socket 939 rig, for example.

An approximate $75 expenditure (notice I did not use the word "investment") and you are good to go until such time as you upgrade the entire rig. That would be my suggestion.