My motherboard has 2 RAM slots. I keep running out of virtual memory. My RAM is already at 526mb. My hard drive is a 320gb Western digital Caviar w/1.5 gb Athlon processor . Can I just add to one of the slots or do both have to be the same mb of memory?

The slots themselves have no bearing at all on virtual memory. Virtual memory is the paging files and are determined by a couple of different factors.
a) usually is set to 1 and 1/2 the physical memory installed... as you have 512 MB of memory then the virtual memory would normally be set at 768MB or higher/lower depending on personal choice.
b)moved to a different location ( different physical drive) as that it speeds up the processing by splitting the files between drives...this only works well on high demand systems and is not recommended for home users at all!
c) reduced to lower size setting so as to force loading faster...this is only for single processing computers like for doing computations and such and again is not recommended for home users.

HowStuffWorks "How Virtual Memory Works"

As to adding to the extra ram....this is dependent upon the motherboard and what it can handle and what operating system is being used...

regardless of the motherboard requirements all 32 bit operating systems are limited to being able to use only 4 gigs of memory and even then may be reduced to only recognizing 3 gigs....This happens a lot in windows XP.
64 bit operating systems are not limited to the specific amount of Ram installed.

You must check the specifications of the motherboard to determine the amount and type of ram that can be installed.

With the newer hyper threading technology it is best to use matched pairs of RAM but not mandatory. It is important to use the same speeds as ram will revert to the lowest speed installed and can slow a systems respnces dramatically!

I am running XP Pro and use Firefox exclusively. That answered my question.