You might have some trouble with swapping out the motherboard in that dell.To tell you the truth,it might be easier,sometimes cheaper,to build a new computer.The hardware in the computer is too old,they dont make socket 478 motherboards anymore.
http://support.dell.com/support/edocs/systems/dim4550/techov.htm#1101565
Here are some diagrams.Im pretty sure the PSU in that dell is missing 2 standard pins,so you would probably have to buy a new PSU,but the PSU's in a dell arent the standard size.Also the front headers arent standard,they are dell's own design.
I'd build a new computer.