I have a Geforce2 in my old/backup system and before I decided to build a newer system I bought the ATI 9600 All-In-Wonder Pro. I used that in my old system until I bought everything to build a new system. Then I moved it into my new system.
I really think you should just order all new parts and build a new system and leave the old system as is. Using it as a internet backup like I do with mine.
My .$02
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