Virtual PC 6.1 requires a large amount of RAM. For optimal performance, you would need 1GB of total RAM, with 512MB allocated to VPC, leaving an adequate amount to OS X.
It should also be mentioned that 6.1 is the Vonnectix version, even though Microsoft bought them, Microsoft hasn't actually released their own version yet. Wait until version 7.0 and there shoudl be large performance and stability increases. VPC 7.0 is reportedly able to make use of the hardware graphics accelerator, rather than using an emulated one.
Just to note, on my 1.5GHz 15" with 1GB of RAM, the processor is an emulated "Connectix processor" - a Pentium II 296MHz. I have allocated 512 MB of RAM to it, and after seeing how poorly Windows XP Pro ran on it, i uninstalled it and put in WIndows 98 SE. It does pretty much the same thing, but runs MUCH quicker and also takes up alot less space on the hard drive.