I bought an Acer gaming PC off ebay with an ATI HD5970 graphics card for 800€ - but the same offer was available for 700€, I just missed it.
It currently does ~580MHash/s and consumes about 300Watt - not too bad compared to other miners, it seems to me. Also, if it survives the mining, maybe I can one day use it as a gaming PC if mining becomes unprofitable.
I was lucky that a friend offered to host it in his cellar, though - would have been too noisy for my flat.