Whole drive use vs 2 partitions for performance?

Here is how I have my system setup. I have 2 partitions on a 320gb HDD, 1 for apps/os which is 50gb and the second as a secondary means for archiving which contains the rest of the space on the drive. My primary means of archiving is on an external hard drive which is always off except when I need to access the data on it which is very seldom. I transfer data from the second partition of the internal drive to the external drive once a month or around that. I do this just in case something should happen to either drive that I have a secondary backup.

Now for the question....

If I strictly use my main partition for apps/OS then will I basically gain in sequential read speed since the drive doesn't need to look beyond the first portion of the drive? I've done some HD Tach benches and on the chart it shows a constant read speed from 0-80gb then dips fast from there on to 320gb.

Also will seek times be less since it only looks at the first partition only?

Yes, as you pointed out the first portion of a drive performs better than the later portions. The main detriment would be if you're trying to access partitions A and B at the same time, in theory your drive will either skip back and forth between the two (thus negating the performance advantage) or it will go A, B, C and you could go fast slow fast. Obviously if you're using partition A and copying from B to your external, the performance on both will slow down.

However if you use partition A 90% of the time and only touch B on occation you'll see a nice improvement. I'm not quite sure how your performance would be if you're playing a game / working on A and playing music on B though...