DB6,
Ironguts & FL310 pretty much have it. I have upgraded many times - now there's nothing left of the original! I have always found that the "upgrade a generatiuon" modifications (adapters on the motherboard, plug-ins have been almost as pricey as judicious buying of new main board, processor and RAM. With the added concerns of comaptibility I've always voted for "clean sweep". With the prices of main boards, RAM (still fairly cheap) and a <<moderate>> processor, it shouldn't cost too much.
I wonder what your applications are? I still run a Pentium 133 at home for a bit of MS Word, Excel, CorelDraw and email/internet, and still happy with that (could be faster but not a real problem) and I have a Pentium II 450 in the office (BIG Corel's and bitmap graphics - still not realy fast enough!!) Modern games really do stress the processor though!
Not sure I go with the generalisation that "Intel for business, AMD for games" is quite true, but the route to take is definitely to change board processor and RAM. If you go for a "moderate" processor, try to ensure that you can swap for something a generation higher at later date (though the processor manufacturers seem intent on blocking that route with a different board requirement for every new processor!)
Asus and Gigabyte boards are what we use here - really compatible and flexible
Good luck!!
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In the middle of upgrading two ponds in the garden