Well in the end I put my max bid at €56 (about $75) and got outbid by one person... Oh well. Am actually thinking I do need a duel processor so I can have 64bit windows but my budget is tiny. I have to get the whole thing with screen and all the little pieces for under $200 and in Europe prices are a little higher than the US :/
Just wondering, how should I judge a CPU? I mean... is 2x 2.5ghz = 1x 5ghz (i know that doesn't exist)
It is tough to judge a cpu without knowing exact details and generation. 2*2.5ghz does not = 1*5ghz... It could be close if the chip had the same number of transistors and one was running a dual thread computation while the other was running a single thread.
What I can tell you for certain is that a newer generation cpu that has the same clock speed as an older generation will for certain be faster, also in most cases it will likely use less power.
Since your budget is low and you need everything, you may have to shop around for a while to get a good deal. I feel it would be worth your time and effort to do so. P4 cpu's are quite dated and would struggle with a lot of modern software and websites.
As I mentioned I would be looking for at minimum a fast core 2 duo or core 2 quad. If you can find an older i5 or i7 similar to the 2 I have posted with everything you need and within your budget I would go that route.