Can anyone suggest a reasonable upgrade from an ATI 9800 Pro that won't leave me unable to feed my children for the next year but still provide a substantial performance boost?
No question -- GF6600 GT.
You can get a 128MB version of the card for $130-140 from Newegg, and it's available in AGP with no problems. Note that the lowest price card they have listed requires a rebate... you can pay an extra $8 for not having to deal with the rebate hassle.
And no, I don't believe a 6800 or 6800GT is worth the additional expense... although if you decide that you must
have 256MB of memory then the best choice is certainly a 6800GT (around $250 after rebate).