The fill rate of a 6600GT really means that the 256MB version does'nt really provide as much of a performance boost as you would like to think.
The 8 pixel piplines mean that the pre-rendered frames can not be drawn any faster and in effect sit in the memory.
You would be better of sticking to a 128MB version or upgrading the calss of GPU you are think of buying to 6800.
The 6600GT is by far the better of the cards you have mentioned and click here are selling a 6600GT PNY card for £105 inc vat. click here
The 6800GT's have dropped below the £200 mark click here would really be a jump up in performance. Anything above that would not really be worth getting due to the low specs of the rest of the system.
Either a 6600GT or 6800GT would allow you to play all of the game mentioned at very acceptable image quality and framerates.
Would check to see that you powersupply is at least provding 300 true watts to the system as the cards do take a lot of jiuce and anything under 300w could result in powerfaliure and an unstable system.