First, is this version of Vista 64bit? I presume from the amount of RAM but I hate to do that.
Second, if you were not already recording to the external drive then you need to start by doing that. Internal OS drives are just for the OS and program files.
Third, make sure all of your networking is turned OFF and disabled.
Fourth, download and run DPC Latency Checker here:
If you find latency spikes then you need to tweak your system until you find them. Vista can be used very successfully to record though the preferred versions are Business and Ultimate. The Home version has some limitations with regard to controls and what not.
The Sapphire is far superior to anything M-Audio makes so I will concur with your conclusion that it is likely not the problem. That said, making sure the Sapphire's firmware is up to date (not just the driver) is a good idea.
Lastly for now, you might as well download and install Vista SP2. It fixes a few more things left out of SP1.