I've got a 7 year old dell 9150 with XPmce. The way I did it is:
Got Retail W7, as you get 32bit and 64bit dvds with that and no problem if/when I buy new pc.
bought new 1TB hdd (got 3 as wanted others for backup and extra storage)
disconnected XP hdd and connected and installed W7 32bit on a 200GB partition on 1TB drive on Sata 0 on motherboard. My motherboard will only take 4GB ram max and also couldn't get drivermod for W7 for onboard sound (ancient Sigmatel IDT onboard sound) to enable line in/stereo mix to record with Audacity.
I used free EaseUs Partition manager to make the partitions on new hdds prior to installing W7
I now run both as separate OS's, swapping the sata cables over on back of hdds to make connecting the one I want to boot from to motherboard Sata 0.
I am hampered from easier choice of ways to do it as I didn't want a dual boot manager and dell bios doesn't give me F8 choice of drives to boot OSs.
I have so many programs on XP that because of the licences, Giveaway of the Day freebies that I can't put on W7, I wanted to keep XP for those. But I am gradually gathering more W7 ones now.
I keep all my data on separate internal 1TB drive on sata 2 (and backups go to the 3rd one via usb) so it is all accessible from whichever (W7 or XP) OS I boot into. The other OS then becomes a slave on Sata 1 slot on motherboard.
Not a sophisticated setup but I am happy with it at present as all hdds are visible in Windows Explorer. Won't be long now before I get new Tower and make the leap to all W7 on new one. That means I can still keep old Dell XP working as normal as an extra pc.