Quake III Arena	Source: ID Software

Quake III Arena is a First Person Shooter (FPS) that revolutionized gaming as we know it. Using multiple light sources and having graphics textures that can fill videocards.

Quake III Arena MAX 1024x768 demo001
	Video Card	FPS	Ranking
1.	ATi Radeon 9800 Pro	316
2.	ATi Radeon 9700 Pro	309.6
3.	MSI FX5900U-VTD256 Ultra	338.5
4.	Albatron GeForceFX 5900PV	330.4
5.	MSI G4Ti4600-TD	275.1
6.	nVIDIA GeForceFX 5900 Ultra	338.4
7.	nVIDIA GeForceFX 5900 Ultra (500/938)	334.3

Quake III Arena MAX 1024x768 nv15demo
	Video Card	FPS	Ranking
1.	ATi Radeon 9800 Pro	72.1
2.	ATi Radeon 9700 Pro	71.9
3.	MSI FX5900U-VTD256 Ultra	77.5
4.	Albatron GeForceFX 5900PV	77.4
5.	MSI G4Ti4600-TD	77.5
6.	nVIDIA GeForceFX 5900 Ultra	77.7
7.	nVIDIA GeForceFX 5900 Ultra (500/938)	76.3

Here in Quake III Arena we see that the GeForceFX 5900 Ultra class cards are slightly faster than the ATi Radeon 9800 Pro.

Quake III Arena MAX 1600x1200 demo001
	Video Card	FPS	Ranking
1.	ATi Radeon 9800 Pro	202.5
2.	ATi Radeon 9700 Pro	178.3
3.	MSI FX5900U-VTD256 Ultra	229.3
4.	Albatron GeForceFX 5900PV	205.6
5.	MSI G4Ti4600-TD	140.2
6.	nVIDIA GeForceFX 5900 Ultra	230.5
7.	nVIDIA GeForceFX 5900 Ultra (500/938)	245.9

Quake III Arena MAX 1600x1200 nv15demo
	Video Card	FPS	Ranking
1.	ATi Radeon 9800 Pro	71.4
2.	ATi Radeon 9700 Pro	71.2
3.	MSI FX5900U-VTD256 Ultra	77.2
4.	Albatron GeForceFX 5900PV	76.8
5.	MSI G4Ti4600-TD	68.1
6.	nVIDIA GeForceFX 5900 Ultra	77
7.	nVIDIA GeForceFX 5900 Ultra (500/938)	77.3

At 1600x1200 the GeForceFX 5900 Ultra is about 10% faster. I doubt you'd notice the difference though.