Good news! The PC is up and running again. It's probably been one of the RAM bars that didn't fit tightly in the sockets. I moved the PC recently (from honorable donor to the "Arcade Workshop"), maybe they came loose then. Anyway, as soon as I took them out and reinserted them one after another the PC was booting! Yay!

Really thinking of getting more RAM though. Not sure if 800 Mhz would work in this mainboard:
http://217.110.237.70/Manuals/7204-engl-MS-7204-MedionMD8800.pdfThe manual says it's limited to DDR2 533 or 667.
Regarding the graphics card, I just asked because I read that the GeForce 6 and 7 series cards would definitely not run full fledged HLSL without serious performance issues. If your experience is different, I'll give the 6700 XL it a try. I just don't want to have to test every single game I want to play to see if it works out or not, if that can be circumvented with a 20 $ - 40 $ investment.
Can't buy any of the cards from 404 though, as shipping to Europe would probably cost more than all of the stuff he offers combined!

What do you think of a GT 610? Would that be good enough to handle at least the MAME games?
Thanks for your help! I really appreciate it!