CPUS's aint my thing.
But, the barton 2500 would be better with the extra 256 cache over the 2700.
Raw speed would be the 2700 as it works faster.
Difficult one but go for the 2500 for gameplay. Remember it's also the graphics card that helps a game. So if the card isnt up to it a faster CPU will not help. (well maybe a little)
You could go for the 2700 and get 80quids worth of RAM?
No matter what, you should see an imporovement.