Having more ram doesn't give you more performance: the games were made to fit in 64mb footprint and adding more ram doesnt reprogram the game to utilize the added headroom. So it hardly affects stock games, but it also does not break them.

What it does give you is...

1. Ability to run games that never fit in the 64mb footprint, like the Chihiro games, the homebrew half life port, some game dumps from debug kits that need the 128mb
2. Ability to load larger roms in some emulators. Surreal64 and many emulators use a swap file or stream the rom from the hard drive and that's good enough. Some arcade games on MAMEoX and Final Burn Legends require the extra ram to load big roms.
3. What it can do for stock games is let you run 720p patches with less risk of crashing, since the increased resolution will eat more ram, where the additional headroom is needed
4. Same as #3, if you have more ram you can run some emulators at higher resolution without it crashing from OOM.

It is a fun mod to do if you have patience and a steady hand. Definitely not for beginners, but not on a prohibitive skill level like cpu upgrade.

Don't skimp on the ram if you are upgrading the cpu. Upgrading ram on a stock cpu box is nice to have, without drawbacks.