I'll go against the grain here and say to just keep your system the way it is, and enjoy it for what it is. Power supply replacement (of which I only found out about myself a few days ago) being the exception. At some point, after adding ram expansion, CPU accelerators, etc., it's no longer a C-64. Some questions to think about before you spend money on some of the upgrades above....
1. Are you going to use this for word processing / doodling / etc.? If not, no point in a mouse since GEOS is pretty much the only thing I've seen that supports it. You can turn it into a joystick by pressing one of the buttons (I forget how) but I wouldn't recommend it for games... Likewise for an REU. I don't think there is much use / support for one outside of GEOS, unless you would get some kicks from playing around with it on your own.
2. Do you care if your programs take 15 seconds to load instead of 45? If not, don't bother with a fastload cart either. Not worth the trouble. Of course, I had to wait to "graduate" to a disk drive (from a cassette drive!) and my first modem was 150 bps so patience for computers is something that has never really bothered me much. YMMV.
3. Anything that messes with CPU timing could affect the perf of games. Maybe not. Most people that spent the money on these (that I know) had to bump them down to 1MHz to use them with games most of the time anyway.
Now if you're tinkerer and just want to have the baddest C-64 in town, ignore all of the above. I've always wanted to do the second SID chip mod to one of my C64's, but only of interest to someone interested in electronic music...