Upgrading PC

After pricing parts for a custom PC, I have found out that its too expensive for minor problems I’m having…

right now im running a Dell Dimension 8250

Pentium 4 - 2.4GhZ
512 MB RDRAM

Is there any way I can just go out and buy an ATX MoBo and hook it up to this? Possible a new power supply as well.

I want
AMD Athlon X2 3800+
2GB Patriot Ram

Will a standard ATX Motherboard fit in my Dimension 8250? If so, what extra supplies would I need, like a power supply, etc… :stare: