The X-Fi should be a good choice with latest drivers and BIOS updates (there were some issues with some NF4 boards earlier, but the situation appears to be mostly resolved.)
When I use onboard audio I sometimes see my average frame rate dip by 5-10fps, depending on the game, and its audibly worse even when playing games.
The 7800GT is a very good choice. Ocassionally you will need to turn down the resoltuion, AA, AF or detail settings, but I'm sure you'll find a good combination of settings in most games. As others have pointed out, you can always add a second 7800GT if you like, still have spent less than a single 7800GTX 512 and the performance will be better in 90% of all current games where the GPU is a bottleneck.
The latest generation of games are real GPU/CPU killers. Personally I think some of this is due to sloppy programming, or a misguided use of effects which don't always equal an overall improvement in "graphics" - to use it as an umbrella term. F.E.A.R. is a good example - it looks very good, but trails the much older HL2 in texture, model and level design quality, even though it uses some very cool looking effects. I think the performance trade-offs are poor in this game.
Based on the games you said you like, I recommend you give Call of Duty 2 a shot - great eye candy and an awesome game play experience, very much in line with MOH.
To me the visual benchmark on the PC currently is the single HDR level for Half-Life 2 called 'The Lost Coast.' When I play this at my 24" monitors maximum (native) resolution, I see some slowdown, but at any lower resolution it's smooth as butter and the overall quality of the graphics surpasses everything else, including the other Source games that use HDR.
Good luck with the new PC, I'm sure its going to be a lot of gaming fun!