Ive done a bit more research into my PC (using SiSoftware Sandra) and I don't think my motherboard supports SATA so I will be going for an IDE hard drive, and most likely this one:
http://www.pcwcomponentcentre.co.uk/invt/6y160p0
My motherboard is a MSI (Micro Star International) KT400-8235
I looked at my Port Connectors and it says both my primary and secondary IDE are none - ATA / None. Does this mean they are both empty? Any questions about anything on my pc just ask me and I will try to find it out. I'm appreciating this help a lot!
Also Im almost 100% sure I have SP1 installed, I have all the windows updates installed that I find when I visit Microsoft Windows Updates, but I still looked at my installation history. I have quite a few things that say Service Pack 1 or SP1 after them.
I have a DVDRW drive and another DVD player drive, and 1 hard drive already. So I should have room? - I also have a 9in1 card reader will this be using one of the IDE spaces?
Also you mentioned I will need to partion it for it to list in 'My Computer' once Ive put the hard drive in, will XP auto detect it and I will just have to partion it or what exactly?