I'd go for processor speed on your original configuration. Adding memory is easy to do yourself, and it's usually cheaper to purchase it from a third party source like Newegg.
As for processor speed, it really depends what you are going to do with the computer...how much difference is there between 1.83, 2.0, or 2.16 in the CoreDuo CPUs? From the benchmarking I've read about, I don't thinkg it's all that significant. I'd probably choose 2.0 as a decent value and a little bit extra - but 1.83 is probably just fine for the majority of applications.