I've been toying with that idea as well since I also own a p2-450. I will be building a new athlon system in the next month or two so I would want to get a GF2/ultra for that puppy. I currently own a tnt2u and while I got some improvement using the new 6.16 drivers, it's still a slow card nowadays. So, I am leaning towards a GF1 ddr or the 5500.
ASUS Rampage III Formula Intel X58, Intel Core i7-930 @ 3.6, Patriot Viper II Sector 7 Edition 12GB, Galaxy Geforce GTX 680 SLI, LITE-ON Internal Blu-ray Burner 12X, Dell UltraSharp U2711 IPS-Panel Black 27" 6ms, Corsair Hydro Series H100, ENERMAX SGALAXY EVO EGX1250EWT, Corsair Obsidian 800D, Windows 7 x64
Alienware X-51, Core firstname.lastname@example.org, 4 gb 1333, 1 TB WD, Evga Geforce 650ti