You might just pick up a AMD Radeon HD 6450 1GB DDR3 for around that price (if you are lucky) or just look at second hand choices.
But you are going to have a problem getting Civ V to work at anything approaching an acceptable level, even with all the eye candy turned down, as you are running at 1680x1050 and Civ V is a pretty demanding game.
Having a budget of £25 is simply not going to give you what you need. Then again the GeForce GTS 450 I first noted (over £60) might bring a hole new load of issues as it does require an additional 6 pin plug.
If you get the 2GB of memory and then just a £25 new GPU you will be no further forward than you are now.
If you can have 4GB or memory in that system then great (but it would cost) if not then 2GB would simply have to be ok. That also is quite slow DDR 2 memory.
If you could afford the £60 plus card then great but that might throw up issues with your PSU.
Tough choices