virtualbox running win7 64bit, too slow
A Mac, especially one running Snow Leopard or Lion, requires at least 2 GB of RAM for decent system performance. 

Even through the system requirements for Snow Leopard are 1 GB, it doesn't perform well with 1 GB. Lion requires 2 GB of RAM just to be able to run. If you plan on running a virtual machine setup and expect decent performance from both machines, I'd have 8 GB of RAM in my machine. You can get 8 GB of ram for less than $50 from OWC. 

Also, If you're not doing anything major in Windows, I'd lose the 64 bit installation, bump it down to 1 GB of RAM and try again. I use 32 bit Win7 Enterprise for work through a Parallels VM and only allocate 1 GB of RAM for it and it runs great, especially in coherence mode.