Even if you lose 20-30 MBs of RAM to background services, that won't really affect performance noticeably outside of the most memory-constrained hardware builds. How many games do you run that grind to a halt just because you only have 600 MB free RAM to let them use instead of 630 MB? Having bits of memory in-use as opposed to empty does not slow down access to other bits of memory, unless you manage to fill up the physical RAM entirely and cause hdd thrashing..
Now certainly, ditching huge memory hogs like NAV can net performance gains. Closing off larger programs like Skype and Instant Messengers, web browsers, etc., before you run a game can free up a lot of memory. I just don't see much potential in axing ~1MB background apps, esp. when they are hardly ever run by the task scheduler and get paged to disk.
"As far as defragging goes, I've read posts where people just lose files/programs on disks as over time, XP will write over files and programs."
It's fairly safe to assume such posters had a gross misunderstanding about how their system worked. Its far more likely hdd clusters randomly became corrupted at a physical level than some nebulous claims of XP overwriting existing file segments due to marginal fragmentation..
"That's why they (HDD OEM's) suggest that you defrag every once in a while to allow the OS to rewrite/update the FAT."
They just do that to shift blame away from themselves for less than perfect hdd manufacturing.
It can't hurt to defragment once a year or so, but don't think its some sort of panacea for your system's woes.