Not too sure myself but can hopefully point you in a few directions.
Try Overclockers uk click here
They list a lot of accessories for cooling from bigger quiter CPU fans such as the Zalman range, to liquid cooling. Does your current case have any space for extra cooling fans? If so newer cases are quite cheep on Ebuyer. The quieter PSU will probably help alot.
Overclockers also sell HDD coolers, so might take the heat out of your HDD.
Also consider where you can place you computers. Keep them out of tight corners so air can circulate more freely.
Sorry, this has turned into a bit of a ramble as well. In short you can easily add extra cooling to your current case but depending upon the size of it a newer, larger one could allow a bit more internal space for air circulation. Really depends on your budget and determination.