will my machine be able to take advantage of all that the 6600GT has to offer?
Your CPU's a little on the slow side - OK, a lot
on the slow side
- compared to what's out there, but it should be adequate. Though, yeah, a CPU upgrade should definitely be next on your list, depending on what's the fastest CPU your MB can handle.
Firing Squad has a P4 / 6600GT CPU scaling article: http://www.firingsquad.com/hardware/geforce_6600_GT_scaling_performance_pentium4/
Unfortunately, it doesn't include a 2.0GHz CPU, but it at least gives you some vague idea how much you'd benefit from a faster CPU.
Anandtech also has an article on CPU scaling in HL2: http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2330&p=1