I use workstation 7 every day at work. I have a Linux Mint machine with 2GB of memory that i use constantly for virus research and error research surfing, as well as personal surfing such as this page and other sites and message boards I like to keep current on. My host is a Core 2 Quad at 2.5GHz with 8GB of memory running 64 bit Windows 7 Ultimate. Even with the VM running all the time, the computer still feels fine. The last time I rebooted the VM, the uptime said 105 days. Yes it uses some Memory, but I have the available memory so it does not slow down the computer. I have had up to 3 VMs running at the same time, and still it doesnt impact the overall speed of the Host OS, only if all of them are doing something at once.
Impact of using a VM depends heavily on what you are doing with the VM, and how powerful the host is. Generally any modern dual core processor computer with enough memory to not max out while using the VM will run with little impact to the Host's performance.
Performance will be hit if there is a lot of IO intensive things happening on the VM of course, and especially if the VM is located on the same disk as the host OS. Just a couple things to be mindful of.