Speed will not be a issue, but compatibility with your computer hardware might be, it depends on the age. I found one laptop that would not load windows 7, a message popped up saying it was incompatible.A few more to answer would be, "Will the programs I buy in the future need 7?"
Do I have enough RAM and Speed to make 7 worthwhile?
All of my software works on XP. For now. In the future, that's anybody's guess. And if push comes to shove, I can always install Win7.
Speed was also an (unspoken) thought that I had. In order to best use an older machine's specs, am I better off using XP? I'm thinking right now that I probably will be.
You can download and run the windows compatibility checker to do this. If a windows 7 disk came with your computer, it should work fine. Since you have a new hard drive, try installing it, you may be surprised at how smoothly and quickly it installs. You can always install XP over it, just choose the erase partition option when installing XP, or use a utility to format the drive.