The RAM capacity is theoretically limited by the number of bits of your processor. If you have a 32 bit CPU, then you can't go above 4GB so anything higher will be an expensive coffee stirrer. If you have a 64 bit CPU then you can have silly amounts of RAM so you don't need to worry. You should check the manufacturer's website of your motherboard for the ideal frequency and capacity of RAM it can take. Remember that if you buy RAM sticks of different speeds, both will operate at the slower speed.

Just so you know, whoever told you to defrag and virus scan to fix your internets is talking out of their ass and has no idea. For the record, RAM won't fix it either, but I suspect you're aware of that already.

To elaborate on ValiantKnight's notes on defragging, think of a hard drive like an old vinyl record player. Imagine it trying to play a record that had bits of song scattered across the whole disc. The arm would be jumping all over the place, right? Well defragging is analoguous to getting all of the bits of song and putting them together like a normal record. The ostensible end result is (slightly) faster access times and (slightly) less wear and tear on the drive. It won't improve the speed of your CPU though, only the time it takes to read data from the disk. It won't fix internet at all either.