Well I don't know the non ATI branded cards much, but first thing you should do is identify the core in the card. Aida32 will show you this easily if you go to Display -> GPU. If the card is listed as an "R350" don't even bother. If it is the 360, then proceed to read the sticky thread at the top of this forum titled "Turn your 9800 pro into a 9800 XT".
Personally, I'd probably just buy an artic cooler, overclock it, and call it a day. I've never had a bad flash on any device myself, but why risk it? Yeah you can plug in a PCI video card and flash the origional bios back onto it, but the gains from flashing haven't looked anything as great as the 9800SE->pro softmod.

Bringing the clocks up to/near the XT clocks sums up the majority of the pro->xt mod, unless I'm missing something..Dunno of any features in the XT other than overdrive and the sensors which may not even be on your card to begin with, thus impossible to enable.
If it's an r360, you can likely do it. Some 350s can too but they have to be underclocked making it pointless..
All it involves really is giving it a new bios..Pretty much reprogramming the card itself. Once it's done, you don't need special software for anything with it, other than omega's drivers for improved quality, etc.
Performance increase: Really depends on your rig..If the card is the gaming bottleneck, it will help a lot if you can run high clocks, if you flash it or not. It won't be an x800 by any means, but it can really make or break having good IQ+good fps, at least for me and bf:vietnam. If you have a celeron cpu or something, you wouldn't notice the performance gain likely.
Speeds it will run at: depends on your cooling and some random luck of the draw. Also, the memory brand/type will affect how well it OCs too..read the above thread for some good feedback from others.
Enable more "stuff": It will enable overdrive and the thermal sensors, if they are present but just disabled. As far as I know, *most* pros lacked those even when modded but I could be wrong, or you could be lucky.