You can easily open an xarcade up and change it from 8-way to 4-way stick. This is easy enough to do but not something you would want to do every time you change a game. It will take you a few minutes. So more of a set it and forget it option.
Even with a 4 way, I prefer a d-pad controller over an arcade stick for console games.
I suggest searching amazon or ebay for
"nes usb controller"
"snes usb controller"
"n64 usb controller"
"genesis usb controller"
"saturn usb controller"
Your choice would depend on the games you want to play but snes and saturn would be my first pick with n64 being third.
Other great choices are a wired Xbox 360 controller or
ACT LABS - PowerRamp MITE
Retro Uprising - How to Play With a Game Pad
---------- Post added at 01:18 PM ---------- Previous post was at 01:12 PM ----------
I just reread your post. This sounds like what you want
Amazon.com: Buying Choices: RetroBit Retro Adapter NES Controller to USB Port Adapter
plus
Amazon.com: Buying Choices: NES Advantage Joystick
$30 including shipping and exactly what you want.
There is also a similar SNES option - which would have enough buttons to play most games - better deal IMO.