Since you are coming from a background of building PC's I'll approach the question from that perspective. In the PC world you can buy the parts brand new, and build a PC cheaper or at a comparable price to buying one from a retailer.
This is impossible to do with a Mac
To build a Mac from parts you'd have to either:
1.Buy used or old parts and get exactly the right ones for the Mac you are trying to build. Here you have the problem of the reliability of the part you purchased. In addition, given the way apple's been cutting prices you will probably end up paying just as much if you start from scratch, as you would spend just buying a new mac.
2.Buy new parts from places that stock mac parts. There are a very few places that will sell a consumer brand new replacement parts for Apple computers. The problem is that they charge a high markup on the parts, so unless you only needed one specific part it wouldn't make economic sense to buy all the parts from these re-sellers. If you did for some reason try to buy all the parts new from them, you'd easily end up paying 4-5 times the cost of the new computer bought fully assembled and working from apple.
So, if you are planning on just playing around a little, you're best bet would be to buy a mac mini, refurb from the Apple store. You'll spend about $400, it'll be compatible with the monitors and stuff you use now (Except for PS/2 keyboards but you can buy an apple USB keyboard for like $25) If you don't like it, you can sell it in the marketplace for close to what you paid.
if the mac mini is still out of your budget, just look around for a deal on a used g4 or g3 powermac.
If you want a workstation, a powerfull computer that you can use as your full time system, then you're still beter off just buying a powermac or iMac from apple. if that is out of your price range, a latter model G4 and get a processor upgrade (933 Mhz and up I beleive have standard slot for single or dual processor upgrades) you'll probably want to upgrade the memory as well, which is a little more difficult than PC's but not much, for any G4 powermac besides the MDD (Mirrored Drive Doors) you're gonna need SDRAM, not DDR. I'm not entirely sure which models take which speed, but if you buy PC133 SDRAM, it should be able to run in the PC100 slots on older G4's as well. And the Mirrored Drive Door Powermacs take PC2700 DDR I think.