Could you be a bit more specific? 50-78% spikes or it staying at that useage?
What kind of CPU do you have and how much RAM?
If your seeing the occasional 50% spike soon after boot or while doing normal activities, with a dual core for example:
I wouldnt think is out of the ordinary.
Depending how much RAM you currently have, more can help with the overall feel and responsiveness of the system.
IMHO, 2GB is the bare min. for any Win 7 machine.
And 4GB would be the sweet spot.