Marine's right, the smaller the resolution, the more chance of the problem being a CPU bottleneck. The higher the resolution, it will be a GPU bottleneck.
@OP,
I wouldn't go with the 945. I was going to suggest grabbing a cheap second hand 955 then overclocking that to ~4ghz, but I'm unsure if your board could handle that. The PIIs is pretty much a tweaked PI, so you won't notice any significant performance difference.
The Pentium G840 is only dual core but because of the higher IPC, it will perform better in games, since games usually only use two cores. From your current Phenom you should see quite a jump in performance. You can't overclock the Pentium, but you can always drop in an i5 later on if you want even more performance.
If you look here you can see the 6850 outperforms your 7800 SLi, and at that resolution it should be able to handle most things on max. Your board can't do Crossfire but it still can do single AMD cards with no problem.
With both the G840 + 6850 you'll see an overall improvement in gaming.
@OP,
I wouldn't go with the 945. I was going to suggest grabbing a cheap second hand 955 then overclocking that to ~4ghz, but I'm unsure if your board could handle that. The PIIs is pretty much a tweaked PI, so you won't notice any significant performance difference.
The Pentium G840 is only dual core but because of the higher IPC, it will perform better in games, since games usually only use two cores. From your current Phenom you should see quite a jump in performance. You can't overclock the Pentium, but you can always drop in an i5 later on if you want even more performance.
If you look here you can see the 6850 outperforms your 7800 SLi, and at that resolution it should be able to handle most things on max. Your board can't do Crossfire but it still can do single AMD cards with no problem.
With both the G840 + 6850 you'll see an overall improvement in gaming.