Thank you all for your replies! I think I've decided to upgrade Mobo, Processor and RAM since changing one means I have to change all. And seriously, DDR1 RAM and the AMD Athlon are very outdated anyway. I think the graphics card will still be adequate for a few months. Primarily for SC2 and Gothic 4: Arcania.
Of course, when Diablo 3 comes out, that's a whole different situation. :)