Depends on what you need, if it's just a motherboard, new RAM and a processor, you can get top notch material for about 400 EUR.
GA-MA790FXT-UD5P, the AMD Phenom II X4 955 and 4GB of DDR3 ram is a very good combination.
If you want an entire case: pick WD HDDs, and find a case (Antec or something) with a decent PSU (prefer a rather strong one
)
Also, I would buy a floppy drive, even though it's nearly obsolete, but it comes in handy at time (for example when you want to install XP and you need SATA drivers). It only costs about 5 EUR for a floppy drive anyway..