OpenVZ has settings to limit every aspect of a runaway VPS.

It can limit memory, burst memory, disk usage, disk io speed, cpu access and cpu %.

But how these things are configured and sold is the determinant for performance. Some companies oversell and your server does end up in swap land, slowing everything down.

Swap is done by the kernel and is not restricted by the cpu or io limit numbers that otherwise apply if you were writing a file in your VPS for example.

Burst memory if properly configured and properly used will save you a lot of money. The problem is some people treat it like guaranteed memory, expecting apache or mysql to be able to use it 24/7. Thats insanity.

Burst memory is intended for temporary usage, such as compiling a program or updating your system. You don't want to run a business in burst memory!

Get a VPS thats big enough to hold apache, php and mysql functions under load, and use that burst for when you're setting it up or updating.

More descriptions of VPS terminology: http://www.vpsville.ca/terms