Anyway, now that I have my new MBA rev C with the TS128B SDD, it's amazing how fast the virtual machine runs, and I have not seen limitations in the number of apps I have open, though I don't use that many simultaneously (2 or 3). iStat shows that I only have about 12 MB of free memory when I am running Parallels, but it doesn't seem to matter how many programs I have open. I suspect it is swapping memory to the SSD, but since the read/writes are fast, it's almost like having a lot more memory.
So, if you have access to a rev B/C with a fast SSD, you might try running your virtual machine in Parallels. I don't know if you are running XP or Win 7. Maybe this is a bigger problem in Win 7.