This is the point where it's pleasure and pain.  You simply need to aquire the knowledge necessary to make an informed decision that will best please your budget, and the advice given here is very good. 

I ended up buying on eBay, but I had already decided on the machine, and only needed to shop for a good deal (and an auction I felt comfortable with).  If you go that route, be patient.  Even if you don't buy there, it's useful to watch auctions and see what sort of configurations are selling for in your price range.  In this way a good sense of just what to pay and pay attention to will develop, and perhaps a great deal may be found.  After loosing three auctions because I refused to pay more, I put in a low bid on a great  machine and won.

I went from a Dell 8200 purchased from Dell to a Mac Pro.  I don't regret buying either.  It was a logical step, and I knew that in the end spending more would be worth it.  Don't hesitate to look a bit beyond your budget.  You too may fall glassy eyed at feet of the multi-core kingdom!