I should put this answer on a macro key: it's impossible for us to answer this question.
You need to benchmark and measure. Every situation is different.
If you are constantly running at high CPU utilization, you need more horsepower.
If you are constantly running out of memory/swapping, you need more memory. If your database performance is poor due to high utilization and small memory cache, you need more memory.
If your Internet connection is saturated, or saturated at certain times of the day, then you need more bandwidth.
If you don't KNOW how much CPU, memory, bandwidth you are using, how on earth can we answer this for you?
Why are you running a server on a T1 in your office? (I presume it is in your office.) Is there some good reason this isn't co-located in a data center?
As far as "power" goes, I would go with multiple cores over the very highest clock rate. With dual-core chips overflowing the supply channels, I wouldn't build a webserver today with a single-core CPU. And memory is cheap enough that there needn't really be a tradeoff. Pay attention to the MAXIMUM memory capacity and get what you think is reasonable. You can always add more later, and it will be cheaper than it is today.