It is possible but your going to have to find a motherboard that is the same socket as your current cpu. Also, RAM has to be the same type.

You can change the motherboard to anything you want, that FITS, if you get the CPU & memory for it.  IF you want to continue to use the same CPU and RAM?  Then you need another Socket 370 (Pentium III).  For a $300.00 budget dude, you can buy a nice new TOWER and scrap the old one altogether, or just use it as a MEDIA SERVER for your PS3 or XBOX360.

Please keep an eye on http://www.dodtracker.com/oneaday/ - for PCs as low as 168.00 for DUAL core processors.  Also, you can try PACIFICGEEK.COM for towers.   $300.00 will get you a NEW tower with everything in it, vs just changing a motherboard.

You probably do not have a lot of upgrade options for the CPU, probably 1.4Ghz to 2, for a Socket 370 platform, at 133Mhz front side bus, unless it's 100Mhz clocked for lower speed ram...

With $300.00, just buy a new tower unit, and be at no less than one generation behind.  YOu may be able to pick up an AMD 3x Opteron tower...  U just saw a 4x intel system for 489 eralier today.  Each core at 2.3?Ghz...

I just bought a Athlon64 X2 4200+ for 60.00, but it came with bent pins, so I got a 15.00 refund on top of that for having it fixed.  That's why I have two extra Socket 939s left over.