If you are thinking of keeping your motherboard and upgrading the CPU to a Core2 Quad, what is the memory you have? There is someone else on here with a Core2 Quad using 800 MHz DDR2, and the memory seems to be a bottleneck for the game:
http://forum.1cpublishing.eu/showthr...t=33167&page=2
The Core2 Quad can run DDR3 at 1300 MHz, but it needs a motherboard that can take that memory, and if you have to get a new motherboard and memory you might as well get an i5, the cost won't be much more.
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OTOH, if you're thinking of making changes to your computer as a new thing for you, then by all means go for it. It's not hugely difficult if you take your time and follow instructions. I'm seeing i5 compatible motherboards on Amazon UK from about £50, memory from about £25 for 4GB or £40 for 8GB, and four core i5s from about £150 (there is an old two core i5 for less, that probably won't be as good as the Core2 Quad, and needs a different socket and motherboard).