My site is completely overloaded and I need some advice and help!!

I'm running at Rackforce.com in a shared virtual environment. i'm one of 4 machines on this system which gives me just less than 500 MB of RAM and 25% of a 1700 mhz CPU.

Best as I can tell I have 70 to 100 users at one time ... I've cached every module and block I can find and still TOP tells me that load ave. are 15 to 35.

The site is so slow as to be unusable.

Questions:
1. is there anything I can do before i upgrade to a dedicated box?

2. how can I determine accurately how many users are on my site.

3. if i haev to go dedicated where should I go?

thansk,
-Robert