Not only depends the amount of RAM or the frequency of it you've instaled in your machine, depends too of the CPU usage, drive and registry fragmentation ,etc...
Try this before you buy more RAM modules :
First check the CPU and RAM usage without and with MUGEN running if you have high usage
follow this steps:
* Disable run startup aplications that you don't need to free ram and CPU usage
* Disable services that start with your O.S. to free ram and CPU usage
* Defrag your drive to faster read/write on it
* Clean the O.S. registry to get faster access to it
NOTE: (never install more than one antivirus make the machine slower and may cause conflict between them, one is enough if you want more protection install a security suite )
If you don't know how to do that in your O.S. pm me