Acer Aspire 5750G @ £590 had Core i5 CPU and the best graphics card with the GT 630M.
Lenovo IdeaPad G770 @ £550 has a 17" 1600x900 LCD screen for more workspace/gaming real estate and the HD 6650M graphics card is a close match for the GT 630M/GT540M.
(review was not an identical match in specs)
There really isn't a 'downside' to choosing an i3-2310 / GT 540M model over the i5-2450M / GT 630M.
The CPU is just a bit less powerful but not so much you'd need to worry it can't handle the same workloads as the i5. And it's enough CPU to support that GT 540M. And it's quite a nice savings.
I understand i'll be limited with the GPU. It's not really a dedicated gaming machine, I have consoles so most my games on that. This will be mainly used to play MMOs that you can't get on the PS3 or 360.
Have you used a 1366x768 LCD before?
You might be able to adjust your on screen resolution to 1366x768 and 1600x900 to get a feel for the difference in the amount of workspace. For me - that's important.
But it might not be for the type of work you do.
The anti-glare LCD on the ProBook is a nice advantage if you want to take the laptop outdoors (in the shade), to a coffeeshop or a conference room / lecture hall where you can't control the lighting.
And the Acer gives you the best gaming experiance.