I would not run a database nor on XP nor on W7. Neither are optimized for server tasks. Anyway the OS matters little when it comes to databases - raw I/O speed and memory size matter far more. And here a 64 bit 7 will outperform any 32 bit XP - and even a 64 bit XP which was mostly a "proof of concept". Moreover the 7 kernel is far better optimized to use a large amount of memory and multicore processors (which were not available when XP was designed - IIRC XP came also with an HAL optimized for single core single processor hardware...)
And if you're just running the client running the queries against a DB server, well, again, the OS matters very little....
If your code shows huge differences between the two OS, I guess you have some hidden issues in your code.
Surely not all bugs are fixed in 7, but XP won't get any bugs fixed. And newer apps may use APIs not available on XP.
Biting the hand that feeds IT
