Try out Virtual box, it's free and setting up win 7 and try it out shouldn't take much time.
If you're not happy, then check out either VmWare or Parallels.
I'm using vmware, I simply bought that because I've used vmware before on win/linux hardware.
If it's better than parallels, I don't know, but vmware is working great with both win 7 and xp.
If you are only gonna run one or two win apps, I'd recommend making a stripped xp installation, like tiny xp. Because it will run alot faster and need way less memory.
I do atm have win7 on bootcamp (32 bit..) and a very light xp installation in vmware with only 3 apps. The whole installation uses less than 100 mb ram, and it runs very comfortable on 512mb ram dedicated to windows, I could perhaps lower it to 300ish, but idk
