I agree with koala, increasing memory would help a lot, and not just with the virus scanning, but the overall performance of the machine.
Here is a great non-profit, independent organization that compares many popular Anti Virus programs against one another in real world situations. They also have performance reviews for how quickly some run, which may be what you're looking for: http://www.av-comparatives.org/
Here is the specific report I'm referring to (please note that the date on this particular report is from 12/2009, and AV programs have likely updated and upgraded since then. However, the data is still good for consideration): http://www.av-comparatives.org/image...ance_dec09.pdf