What should I upgrade?

More ram should do wonders. I assume since you say 512 dual channel you have 2 256mb sticks?
You could probably get 1gb of ram (2x512) and a new cpu for about 200 euros. An Athlon XP 3000+ w/ 400fsb and a 2x512 kit of PC3200 DDR from the mushkin or corsair value lines should come close to that. If you dont plan on doing any overclocking the value series ram is fine. I've got 2gb of the mushkin value ram in my machine right now running at 2.5,3,3,7 timings with bios set to defaults. It works great and is dirt cheap. If you want something a little more high end (if you are going to OC) I'd go for the corsair xms line, a higher end set of mushkin, or crucial ram.
In my experience corsair ram is either a blessing or a curse. Make sure your motherboard lists the exact ram you choose as being compatible. Corsair xms works like a champ in my little brothers computer but does not let my spare computer even post with it in. Same motherboard, just different revisions. It seems to be very picky stuff.
When in doubt crucial works in everything.

<A HREF="http://www.folken.net/myrig.htm" target="_new">My precious...</A>

Thank you very much... But the question is... Where am i gonna get an athlon xp 3000+ everything has changed to sempron :S