I am not saying AGP is dead now...but it will be real real soon. Buying an AGP board now is not a wise move at all.
If the OP thinks he can save additional $200 or so for 6800GT (and assuming it will hit that price point in 2-3 months), then he should just wait to upgrade everything at once. Perhaps 6800GT in PCIe will get to that level. Besides AMD plans to lower their processor prices real soon. In that case it'd be worth to fully wait 2-3 months if he can save up for a new graphics card too and buy pcie mobo and pcie x800xl or something for $200. Upgrading to A64 3000+ with 9800Pro will give you like 20% performance if that since 9800Pro is just slow nowdays.
I remember when 9800Pro was top of the line and my 8500 was just 1 generation behind, I upgraded to P4 3.2ghz from 1600+. The performance improvement was minimal since 8500 became too slow by then. Today 9800Pro is almost 2 generations behind (7800GTX is here and ATI R520 will be here). It's simply too slow for A64 3000+. Most importantly sahder intensive games like Far Cry, Doom 3, Halo, BF2 do not respond to cpu speed whatsoever. So in those games the performance increase will be like 5%.