If you get a new motherboard you are going to need a new case and power supply as well. Dell is notorious for using slightly proprietary case and power supply designs. Things may appear to fit but you can run the risk of frying your brand new board. I would honestly stick to what you have, and save up towards building a whole mew box soon.
By the way, did you get a 128MB or a 256MB 9800PRO. Some 9800PRO's can take a bios flash that will make them a 9800XT. My 256MB built by ATI 9800PRO sure did and now I have a fully functional 9800XT. Just another thought...
