I think it is unlikely that an upgrade from a TNt2 Ultra to a Gf2 Mx would yield a very large performance boost with only a 400 Mhz processor. You'd be lucky to pick up 3 or 4 fps. I'd probably try overclocking the TNT2 card first, defraging the HD, overclocking the processor FSB, etc.
A Tnt2 Ultra should still be able to deliver very good FPS in AH with a P2 400. If you aren't getting around 35 - 45 fps in flight something is wrong. I've had a V770 Ultra for over a year now, and for most of that time it was running on a 400 then 466 celeron. I think you're up against the CPU wall here Funked. a TNT2 Ultra shouldn't become the bottleneck until about 600 Mhz or so.
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bloom25
(Formerly of the)
THUNDERBIRDS