Richard already gave you two Web sites regarding RAM. Here's another:

http://www.kingston.com/

Increasing MHz means getting a faster processor, which probably means
getting another motherboard, so you might as well buy a new PC. :-) You
could always buy a used PC that's a few years old. That would just be a
couple hundred bucks and it would absolutely fly as far as you're
concerned! But you can even buy a new PC these days for under $500, so
you might want to consider that as well.

You have a job on your hands:

You need to find out what  kind of memory chips you have installed so
that you can purchase compatible chips that will increase your RAM.
You will also have to know how to open your computer and how to
install that RAM.

I doubt that you can do anything to increase the MHz.  That would
require replacing CPU for sure, and quite probably the mainboard...
and possibly all of your RAM.  That would cost more than the computer
is worth.

The problem with upgrading these is that it can be more expensive than
replacing the system with something new and fast, or used and faster.   This
is partly because of the economies of scale that make current memory cheap
disappear as technology changes.   They don't make so many of the older
styles, so they cost more per unit to make.   And if your system has RAMBUS
memory, it's going to be expensive due to patent fees.

I regularly see used 3gHz, 512 meg RAM systems (with legal OS but without
keyboard, mouse or monitor) in the $200 range.

As you price these parts, look also at places like the Dell sites to compare
costs and values.

HTH
-pk

For Crucial you don't have to load the active x.   You can download
their scanner executable and run it from your drive.  Its basically the
same thing.  I just did it a few days ago, skipping the active x.

Another program is cpu-z from cpuid.
http://www.softpedia.com/progDownload/CPU-Z-Download-6980.html
It will show the current config.  and some good facts about the chips
you have.