Unfortunately, you've told us VERY LITTLE about the computer so it's tough to tell exactly what the RAM limit is.. If it happens to be a brand name machine, then simply click on the link below, then select the appropriate information from the Memory Advisor Tool options.
If it's not a brand name computer, then you'll need to know the motherboard make and model number to determine the amount of RAM allowed.. It would also help to know whether the operating system is a 32 bit system or a 64 bit system.. Basically, a 32 bit operating system doesn't recognize anything more than about 3.5 GBs.. 64 bit systems recognize a lot more.
I am looking at getting a pc but I am curious to how much ram can actually be used with lets say I have a 2.8 ghz processor. How much ram can I actually utilize with that much processor?