As your motherboard already runs an Athlon II X2, there's a 95% chance it'll run an Athlon/Phenom II X3/X4 (i.e. you DON'T need to replace your motherboard, saving you £30-£50). These triple/quad core CPUs can be obtained second-hand for very little cost: £40-£60. Run CPUZ and report back your motherboard model and I'll check it's CPU compatibility for you.
Your 32bit install of Windows limits your RAM to 3GB. Thankfully ED uses only around 1GB, so there's no problem there.
Your graphics card is the key performance bottleneck. While a GTX 650 would offer a huge jump in speed, a GTX 460 or 560Ti will both perform even better (by 25%+), and will be much cheaper: £40-£50. However, both these cards will need a decent power supply: 500W 80%+ Certified with two PCI-Ex power connectors, £40.
Hope that helps
