Well guys, First of all thanks for answers. I was unaware of this pdf to other format for e-readers conversion. I thought that with an e-reader you needed to buy the books in order for them to "look" good in your e-reader, hence my thread.
Luckily, I got a kindle keybord borrowed from my university library, and started to play with it. The pdfs in that format don't look that good, but it's still readable if you put it in landscape mode. I converted one to .mobi and it looks the same as some books that are directly for kindle (some books that the device already had). So yeah, I think this will work well for me.
I'll buy the Kindle 4 since it is the cheapest. Again, thanks for the responses and sorry for my ignorance.