Well, with integrated graphics it's a wonder that the game even starts up!
The best card you can go for (without having to change the PSU) is a GT240 or HD4670, which is ~£50-60 - see click here & click here (if your order is just under £50, add something for a low cost to take it over the £50 price to be eligible for free delivery. Also change delivery mode to Super Saver at checkout).
Btw, going for a more powerful card will most likely need a PSU upgrade as well, as the graphics card will need separate power from the PSU to function. I don't think your PC's PSU has the requisite connector or enough power for an adapter to be used. G