One good way to speed up VMs is to place them on a separate physical disk. Since you're running on a laptop, you may have to look at getting an eSATA ExpressCard Controller. (I wouldn't bother with USB as the bandwidth just isn't there).
I was having similar issues on my development laptop and after moving to an eSATA setup, I'm very pleased.
When running everything off my single boot disk, I'd see very noticable performance degradation when running particular VMs. VMs running WinXP seemed to work fine, but the minute I powered on a Server2008 VM, the performance would noticably drop. After switching to an ExpressCard/eSATA setup, I was able to run all the VMs without issue.
The other advantage of using eSATA is that you can get a full-size external enclosure which lowers the cost of the drive significantly and increases your options. For example, you could put a 10k rpm drive in there for some really decent IO, or simply have terrabytes of space which you just couldn't get with a standard laptop configuration.
One thing to watch out for though is how your ExpressCard bus is actually implemented in the laptop. For some reason my setup peaks at 100Mb/s when I know the drive natively is capable of at least 200Mb/s..