Ok, so thats something I needn't worry about. Are there any other categories that I should worry about?
Also, I dunno if TigerDirect Ships to The UK. Do they? And If not is there a supplier in the UK / Ships here?
Do not buy ECC ram, its pretty much only for servers. Normal ram either wont mention it, or will be called non-ECC ram.
I don't know who delivers in the UK, sorry. But my preferred online shopping site for computers is newegg.com, slightly more expensive than tiger direct, but customer service, delivery speed and reliability is top notch. Like i said, no idea if they deliver in UK or not.
edit: and one more thing... Not all computers support all configurations of ram. Your best bet is to determine your motherboard manufacturer and model number and look on their website for tested compatible RAM. Anything on that list WILL work. You can also check the limitations of your ram, for instance maximum size (most computers support up to 2 gigs per stick, some less or more) and speed (the Mhz, not DDR multiplier). Anything that matches less or equal to those limits (and matches the type of course) SHOULD work. FREQUENTLY you can use a higher Mhz stick and it will work at the max speed the motherboard allows, I would only try this is you have a hard time finding the previous options.