most of the more recent games are 128mbit i believe. so if you are gonna get a 128mbit card, expect it to hold 1 game, maybe 2. older games like advance wars, chu chu rocket, etc, are 32 mbit so you can fit 4 of those, but its rare that you would choose to have a bunch of older games as opposed to 1 or 2 new ones.
if you are buying brand new, i would definitely recommend getting at least a 256 mbit cart, since it seems like the current max for games. a 512 isnt much more expensive and may be more cost effective.
i would advise against getting a game wallet however. it doesnt add too much functionality to your card: i believe it requires specific f2a cards which are getting harder to come by, its kinda clunky so you wouldnt want to carry it around everywhere, and it does the same thing your pc would do, except that you store the roms on a flashcard than on your hd. if they had something smaller and more of a direct interface with the flashcard, (which doesnt seem likely), then i would spring for it, but unless you are stuck on a roadtrip with no access to your pc for weeks, i would put the money towards a better flash cartridge.