you are correct in that for your applications XP is fine and probably W2000 would do too. Same for the Linux OS you have. But the same I could say about a 1975 car which did the same as today's cars, bring me from A to B. But you probably find 1000 more reason here why Windows 7 is useful and why updates (whether as new OS or SP) are needed.
- Windows 7 (and vista) have better Taskamanger taking advantage of multi-core CPUs, even in single-threaded software. for some of my software thsi is 20% faster than in XP due to using multicore better. You need to consider, XP was designed in a single core 32-bit world.
- 64 bit really works well
install automatically (for the most part)
- with new wide screen monitors aero snap makes working on multiple documents more productive, almost like dual-monitors
- inherently safer (yes, even if XP uses MSE, Windows 7 with MSE still is safer)
- better with SSD (TRIM)
So you see with the progress in technology there is a good reason to upgrade. Of course, the competition of Apple and Linux forces MS to upgrade as well. Again, when you have an old existing system and are happy with it, XP is not a wrong choice. but for newer hardware, sophisticated productivity applications, Windows 7 is much better.
When you buy a new PC with OS, you pay for the OS either way. Then you might as well get the most up to date one :-) It costs the OEM $ 30 to buy the XP license, to buy a Vista license or to buy a Windows 7 license (I'm making up the amount, but $ 30 s a good guess)