Just get whatever you can afford. If it’s not going to serve pages to the public, or if you’re not doing graphics design or anything like that, then there’s no reason to spend any more than you really have to on a web development box.
Although I do more than just web development, my primary workstation is an old AMD Sempron 2600 and it handles everything I throw at it just fine. My testing server (which doubles as a file server) is an Athlon 750 (that’s MHz, sir!) which is almost a decade old and still works great for testing PHP and Python. As a matter of fact, I prefer to test on older hardware because if a script or database query struggles on that box, I can be sure that it’ll do the same on newer hardware when it gets hit by a bunch of users all at once.