I'd compare the 6800 Ultra with the X800 XT. Note- XT, not the Pro. The X800 Pro has only 12 pixel pipelines compared to the XT's 16, which, coupled with the sufficient system, will make a huge difference in demanding games like Doom 3 and Far Cry...
btw, the 6800 GT is virtually the same card as the Ultra, only (depending on manufacturer) having a slightly downclocked core clock speed and memory bandwidth.
AMD X2 4400+ Toledo 1MB/Asus A8N-SLI Deluxe/2GB Kingston HyperX/SLI Gigabyte 7800 GTX 256MB/Creative SB Audigy 2 ZS Plat w/ Creative Inspire 6.1 speakers /180GB Hitachi PATA/19\" AG Neovo F-419 LCD/OCZ PowerStream 600W PSU/WinXP Home SP2