Just get the 2 systems benchmarked, side by side, with equal settings (res, quality, detail, etc).
Half-Life 2: Lost Coast and Counter-Strike: Source both have Video Stress Tests you can use to benchmark the two systems.
Half-Life 2 favours ATI cards too, but with such a huge difference between the cards: Radeon 9550 is well under 9600 Pro, which is well under [about half the power of] the R9800 Pro. The GeForce 6600 GT and Radeon 9800 Pro are 'on par'. Except in Doom 3. However in Quake 4 they are on par again.
3DMark '03, '05, '06 is also fairly accurate.
He won't be getting a decent ATI card for US$100.
The GeForce 6600 has only half the power of the GeFoce 6600 GT btw. Ditto with 6800 vanilla vs 6800 GT/GS.
If he can't get a Radeon X800 XL/GTO is is better off going nVidia and running with FSAA disabled and 4x Anisotropic filtering IMHO.