Taking into account that you already have a screen and/or a harddrive, you can easily buy computer parts for 450€ to self-assemble a quite good computer.
First, find your local computer reseller of trust, check their website and compare to other resellers for the cheapest offer.
Things you should buy:
AMD X3 425-445
I would buy the amd x3 445 as it offers 3 cores @ 3.1 Ghz compared to the 435 @ 2.9 Ghz
and it only costs 2-3 € more and with a little bit of luck you will
be able to unlock fourth core and/or the L3 chache, as amd x3 processors
are phenom x4 processors with 1 disabled core which might be broken.
I can unlock a fourth core on my amd x3 435, but it isnt stable.
Why AMD and not Intel ?
Intel does offer better processors, but those processors start at a price range of 200€ +
So Amd has a better price/perfomance ratio, and the difference to intel in terms of performance is very little, so thats ok.