Try this howto -- down to where it says "install root v7". Instead you will use the radioactive froyo file, but the process is the same.
There are a bunch more howto's for the moment
here.
In short, you run the sprint updater app (version D03) from your computer and follow the instructions. Then replace a .tar file in the installer (see instructions above for exact path) which will give you root access ("rooting" the phone).
Now the phone is rooted and will appear completely stock. The difference is that now you can install a custom ROM.
After installing and booting into radioactive froyo, everything will look the same except for minor differences - the sprint apps (tv, nascar, nfl, etc...) are gone and the phone should run much faster.
Try running the benchmark app 'linpack' before and after to see the difference.