Hi are there any 7500RPM internal HD's for mini's? cuz i want one any links please thanks

AFAIK, the Mini just uses a standard laptop hard drive - and the number you're probably thinking of is 7200 RPM. That shouldn't be too hard to find, once you're looking for the right RPM speed . The only one I found on Newegg is this 40 GB Hitachi, though.

One thing you need to remember is that a laptop hard drive has a much smaller area than an equivalent desktop hard drive, therefore the information density is higher. Because of this, a 5400 RPM notebook drive is going to be faster than an equal sized 5400 RPM desktop drive. I upgraded my iBook from a 4200 RPM 10GB drive to a 5400 RPM 40GB drive, and there was a huge difference in performance. The mini, as far as I've heard, usually comes with a 4200 RPM drive. You might want to look for a 5400 instead of 7200 RPM drive for it, as it will save you a bunch of money, and you are likely to find a little larger drives in that speed.

I heard that it is difficult to change it

It's easy to change, once you take the cover off without breaking the clips .

Hiya, a quick question for you guys, I'm going to look over the service guide I downloaded and printed for the mac mini later, but is there any maximum size that the mac mini will take??

i don't think there is. you can get a 100gb 2.5" drive and put it in there or an ultra fast 60gb 7200rpm one

The 1.25Ghz Mac Mini's come with a Seagate 5400rpm Drive default.
The 1.42Ghz Mac Mini's come with a Seagate 4200rpm Drive default.

you would think the 1.42Ghz mini would sport the faster harddrive..

Right now, most 80GB laptop hard drives are 4200 RPM, and 40GB drives are typically 5400 RPM, which is why those are the speeds in the mini.

Seagate 100GB 7200rpm Ultra ATA100 notebook drive

and

Seagate 100GB 7200rpm SATA 1.5GB/s drive

my mini has 5400rpm, and i recently have just bought an external firewire hd case and put a 7200 maxtor on it. now the mini boots up off the firewire hd, but i think there is no performance difference in using it. is it because of the limited bandiwth of the firewire?

Not really. Go back and read what I wrote before about the physical size of the drive making a difference. A 5400 RPM 2.5" drive is going to have higher performance than a 5400 RPM 3.5" drive of a similar size since the information stored on it is more dense. Not quite as high as a 7200 RPM 3.5" drive, but so close that you probably wouldn't tell the difference in most cases. The spindle speed isn't the only factor in drive speed, either. The buffer size and seek times are important too. It may be that your internal drive has an 8MB buffer and your external one only has 2MB.