Nope, you can't always help changing situations(games, drivers) and if it was good for the price when you bought it, and served you well, you did well buying it. But way back when Oblivion and NFS:Carbon came out we did discuss the trend of the X19xx architecture really shining and how that it may very well be a sign of things to come. (Grape, Cleeve, GW, etc. would remember these talks here).
But, Like I said, I bought two 7900GS's at great prices(for testing and builds) and had an evga 7800GT CO in my main gaming rig for a short while (until seeing it blew at Oblivion so I quickly moved it to a spare rig and bought a new card). I can tell ya, having spent alot of time in Oblivion on a 6800U, 7800GT, X1800XT, X1950XT, and 8800GTS, the jump from 7800GT to X1800XT was huge. Much much bigger than from 6800U to 7800GT.
Anyway, before calling it a night I wanted to add this for other people reading this....... (currently at Newegg) With X1950 pro at $105 AR, 8600GTS $115 AR, HD2600XT $85 AR, And 7900GS $100 AR, the 7900GS is my last choice of the 4 offering the worst bang for the buck for the games mentioned in the OP. And yeah, if you can swing an HD3850, 3870, 8800GT, then go for it.