There's a confusion on the two versions of bootcamp. One is the one found within both leopard and snow leopard called bootcamp assistant. That will create the partition for windows and setup the bootloader so you can install xp, vista, or windows 7. Then there is the "bootcamp drivers" which is found on the leopard and snow leopard dvd's, which is installed within windows. So, you can simply upgrade your vista to windows 7, or do a clean install, the choice is yours. Once windows 7 is installed, you'll need to boot up into windows and stick in a snow leopard dvd for the drivers. This will install bootcamp drivers 3.0.1, which will give you basic windows functionality. Then run apple software update in windows, which will upgrade you to bootcamp 3.1, which is more compatible with windows 7.
So in short, yes you can keep leopard, but install windows 7 and use windows 7 drivers found off the snow leopard dvd. You could even remove the existing windows partition, and start over from the bootcamp assistant within leopard if you want to change the partition size. However this will delete all your windows files.