The P7K500 isn't especially quick, The least expensive thing you could do would be to move it off onto a 1TB Samsung F3 for around £43
For around £64 you could set up a RAID0 of 500Gb Samsung F3's which would give you pretty good sequential transfer speeds. Make sure you have a regular backup system in place though.
Above this budget it's time to start looking at SSD's. £80 gets you a corsair Force 40Gb for your OS and apps, and you can keep the games and media that won't fit on your existing drive
The next budget step would be to stay with the F40, but put a 1TB Samsung F3 in to replace your hitachi. This would cost around £113
After this the steps are simply increasing the size of your SSD to fit your budget, and give you more space for games (even if you are using steam, there is a simple way to have some games on one drive and some on another, using Symlinks).
The Intel drives, Crucial C300, and Sandforce drives (OCZ Vertex2, Corsair Force, Patriot Inferno etc) are the ones to go for.
Personally I'd go for one of the new 120GB Intel X25-M's which are available for about £170 if you shop around (get these in stock OCUK!). This is probably enough for your OS, apps and a good number of your most played games. (I'm using around 90
GiB of my 160GB (149GiB usable) array at home).
With a Samsung F3 1TB for the leftovers you'd have a very speedy system for a budget of £215
Like I mentioned above, a backup methodology is quite important. Something like a two bay NAS with a couple of 1.5TB 5400rpm WD Green drives in RAID1 would probably be fine.
