I'm not sure what the "421 of 639" is referring to. If you go to About This Mac under the Apple Menu, the total installed memory should appear on the dialog box.
You'll also want to know how much free space is available on the hard disk. If you click on the More Info button in the window that appeared in the previous step, then click on the Storage tab, you'll be able to see how much free hard disk space you had.
Typically, system slowdowns are caused by a lack of RAM memory. To know if this is the case, your best bet is to go to Applications --> Utilities --> Activity Monitor. What you're interested in is how many Page Outs you're seeing. Here's what that window might look like:
I've put arrows next to the Page In/Page Out numbers.
I hope some of this helps.
Mike is referring you to Activity Monitory, Apple has provided a useful article on how to effectively use it. You can locate it at: