As a software developer, I've had ample opportunity to compare XP, Vista and 7 in both 32 and 64-bit flavours on the same hardware. Obviously this penalises the later OSes because the hardware is restricted to something that XP can deal with. On the other hand, if you are a corporate IT manager with an installed base of XP machines, this may be the hardware you are starting with. With this caveat in mind...
7 is fairly usable on XP-level hardware. That makes a wholesale migration of a corporate environment from XP to 7 possible. Whether it is actually worthwhile depends on how you rate the cost of supporting a mixed environment, but the choice is yours.
Vista offered no such choice. It is unacceptably slow and unstable on anything that XP will run on. (It has about twice the disc footprint after installation, about twice the RAM footprint after boot, and everything goes at about half the speed.) If you are unfortunate enough to have been lumbered with it, you should definitely consider paying for the OS upgrade because it is probably cheaper than any hardware upgrade that would deliver the same performance improvement.
The 64-bit versions of each platform feel essentially the same as the 32-bit ones. The difference in performance between the two is much less than the difference between XP, Vista or 7.
Now let's consider that caveat. It is sad that 7 can't match XP on XP's own territory, but I have no doubt that 64-bit 7 will knock the stuffing out of 32-bit XP on a machine with over 3GB of RAM and that's probably what it should have if you buy the box today, so for new purchases I think 64-bit Win7 is a no brainer unless you need unusual device drivers. (BTW, if you do need unusual device drivers, do not assume that the 32-bit driver you have right now for XP will run unchanged on 32-bit Win7.)
As a software developer, I expect most of my customers next year to be running XP32 or 7-64. I'll support the other 4 permutations, simply because once I've covered these two, the others come pretty much for free, but I'll feel sorry for anyone lumbered with them. Especially Vista. Comparing Vista and 7 on the same hardware is truly shocking. How could they have let it out of the door?