I was wandering if i can change a workstation Xw6000 (Xeon) processor with a Intel core duo or a Pentium 4. If not what gaming system can i get that is under 300 dollars
That system uses an 845 chipset so it wont support anything better then a pentium 4. For 300 you can go AMD.

ASRock M3A770DE AM3 AMD 770 ATX AMD Motherboard - Retail

OCZ Gold Edition 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1066 (PC3 8500) Dual Channel Kit Desktop Memory Model OCZ3G10664GK - Retail

AMD Phenom II X4 925 Deneb 2.8GHz Socket AM3 95W Quad-Core Processor Model HDX925WFGIBOX - Retail

Total 283.97

These prices are from www.newegg.com most likely the cheapest you can find unless you live near a microcenter.

Most likely you can reuse the power supply and your hard drive and that case you have. The power supply in the one you have now is a 20pin connector it will work on 24 pin mother boards if you plug it directly in the center of the connector on the motherboard. You can also buy an adapter to get you by until you can get a real power supply.