And I have found a spare single 2Gb stick of unbranded ram with different timings but same voltage and Mhz.
Also will this stop the memory from running in dual channel if only 3 of the slots are used and if so what performance gain would I gain or lose?
This would bring my system up to a total of 4Gb.
Any help would be much appreciated
All your RAM would need to be running at the slowest timings and slowest speeds.
so if the memory you have is running at 4-4-4-12 and the stuff you found is 5-5-5-18, you would need to set manual timings in the BIOS to 5-5-5-18 otherwise errors will likely occur.
Yes, the memory won't ALL run in dual channel unless PAIRS of slots are used, you would actually likely see less performance with your change or at least not notice a positive change. You are better off keeping only the 2 matching memory or buying 2 more sticks to match what you already have.