And I was just wondering, (since I'm not really tech-savvy when it comes to the hardware) if I buy more RAM, would I just be able to add it onto my existing RAM? Or does it replace the existing board?

  Because nowadays, 512MB just isn't enough. I want to be able to play Spore when it comes out, and 1GB is the minimum. And it's the same for a lot of the good games out today, besides things like WoW and Sims 2 and the such.

  And, do you guys have any favourite places you like to buy your computer components from? (In the UK)

  The last thing I'd want to end up doing is wiping the boot.exe program from my computer and tanking it by accident, or something...

You can buy RAM from anywhere, its not expensive enough that you need a special site to save money

RAM is definitely a good investment. Get 2 GB at the least, and 4 if you want a lasting upgrade.

You can play Spore on 512 (and any other game with 1 GB requirement) but the performance will be worse.

You can just add it on. Take the stick, open two pins, stick it in, done. Easy. You just need to check the BIOS to make sure it works (and is configured properly, for some PCs). It can't screw up your computer unless you interfere with your old ram stick.

  xruntime said:

"You can play Spore on 512 (and any other game with 1 GB requirement) but the performance will be worse."

Some games will restrict you, but most games I've encountered do not stop you from installing just because your memory is low.

You can add onto your RAM but chances are that you have 2 RAM slots and they are both occupied by 2 254 MB sticks. Your better off buying two 1 gig sticks and throwing out your old ram. Make sure that you get compatible memory, check if your computer takes DDR 2 or 3, 3 is faster so buy it if you can. Newegg.com usually has really good deals and rebates on RAM. PM me if you have any questions.

I think it is time for a total system upgrade!

If you have 512 mb, you definitely don't have DDR3 ram, but Foil's point about making sure you get the right ram for your system is a good one.  I would recommend 2 gig for now.  You can't actually use 4 GB unless you are running a 64-bit version of windows, your system will only see about 3.5 GB or less of it. 

If you have one 512 MB chip, you can buy a second one and add it to get 1 GB.  If you have 2 x 256 MB chips, you'll need to replace them unless you have 4 RAM slots.  RAM is cheap, depending on the brand you can get 2 GB for $30 or less. And given the weakness of the dollar right now that's probably about 2 pounds. :) 

"I think it is time for a total system upgrade!"

You're lucky, because memory is cheap.