For XP (God forbid if Vista) 256MB is really small and an increase to 512 would make a difference to overall performance. However Virtual Memory is different to RAM and this is the amount space set aside on your HDD (swapfile, a.k.a. pagefile.sys), this should be set for windows to manage as that should allocate enough for its needs.
I assume that the missing 60MB is dedicated to the on-board graphics, which further impacts on your OSes ability to work effectively.
Why do you want/need to run a boot-time scan ?
Unless there is a specific issue that can't be dealt with in windows normal mode then you should run scans in normal mode. As another aside, in theory you should bump into the same Virtual Memory issues in normal mode.