Yes, with 8GB available, you can give XP Mode up to 3.25GB.
However, I must stress, you'll get reasonable performance, but it won't be as quick as a native install of Windows XP SP3.
But as I said, you've paid for Windows 7 Professional so it defeats the object if you don't then give XP Mode a go.
If you decide you don't like it, just uninstall it.
However, there is an alternative and that's VMware Player: VMware Player: Run Windows 7, Chrome OS - Free Download for a Virtual PC