That said: I can only assume that you are standing so close to the problem that you can't appreciate the sheer scope of your legal peril. The odds that you have a case are not worth the legal fees to calculate. Lawyers' calculators are not optimized for small numbers. Save your money and change the name. Unless your promotional materials include a fleet of blimps.
If you do hire a trademark lawyer, your money would probably be better spent by having him or her help you with your next name. I'll do my part by suggesting a list of names you should avoid in the future:
Sitecrosoft
SighBM
Sicombinator
Sapple Computer
Siteman Sachs
Sintendo
The lawsuit sounds pretty bogus based on what you've said. Surely Google can't be allowed to have exclusive use of "oogle" just because it sounds like part of their company name. That would be like Microsoft saying no one can have a company ending with "soft".
That said, I'd say this would be expensive and time consuming to fight. May not be worth the effort even though you may be in the right (ethically if not legally).
Is there any chance this could get you into the press? Small company being picked on by enormous corporation. Could be good for you.