The newer Adobe Flash are bulkier than Macromedia Flash 8.
You're better off with Adobe CS3 vs the newer ones.
You'll have to update/patch it so the brush tool doesn't lag.
If you're looking at CS4 it's a little worse off, I never bothered with it's new and not so useful features.
You can pick it up really cheap though, people sell it cheaper than Macromedia8, CS3 and even CS5
You'll find yourself lagging every time you save your file no matter how much you've updated or patched your version if you get CS5.
My only complaint about CS5 from when I tried it on a not top of the line computer is that it can crash while you're trying to save a file over 60mb resulting in all your hard efforts becoming a lump of corrupted data that can only be restored by decompressing your SWF file if you even made one of your movie.
Otherwise the smoothing tracking for the brushes are really good.
Edit: you can get a non-pro version for about 20% less, though you wont be able to use it for money.
I began using CS5.5. I haven't had a single crash yet. However I think macromedia 8 has a better interface (probably because I used that for about 3 years)
Also WHY IS IT SO FUCKING HARD FOR THEM TO ADD A FUCKING DECENT BRUSH SIZE MECHANIC (like most programs such as photoshop)
seriously why the fuck have they limited us to those preset sizes this whole time.