If I were you, I would get the Athlon 3000, it's not only cheaper but it's proven to perform better in gaming and media just as you want. Just to let you know, your ram is capable of 400mhz FSB. This means it will run at 400mhz FSB IF you have a 400mhz FSB processor. If you get the Barton 3000+ your ram will only run at 333Mhz because the 3000+ is only 333mhz FSB. There is a few 3000+'s out their with 400mhz FSB, but i have rarely seen them and when I do there on ebay and go for around 200 bucks.
A gig of ram is pretty much a waste of 500+ mb's of ram because you'll never use it unless your a multitasker. You should think about waiting on the upgrade until you get the money for the ram, and intead of buying the 512 ram that you most likely wont need anytime in the next 6 months, put it towards the 215$ 400mhz FSB 3200+, then you can run your ram at optimal speeds instead of having all that extra ram for nothing. If you decide to go with AMD as I have advised, i would recommend the m7ncd pro motherboard from newegg, its only 64 bucks and supports all the latest features including dual 400mhz ram so when the time comes that you actually need a whole gig of ram, you can buy a pair of 256 400mhz dual channel sticks, because with a good vid card, the 3200, dual ram and the mobo to support it all, you will be a happy happy gamer. The reason i recommend all this, is because I have the same setup (with only 512mb of dual 2x256mb ram) and I run every new or popular game flawlessly with no lag.
This is not advice from a professional pc builder, but merley a gamer that wants other gamers to be able to play games! GAMES RULE WOOOOOO!! cant wait for half life 2 :-p