Hi Vasan,
Applying UML and Patterns by Craig Larmen is an excellent book, and I highly recommend it. I passed the IBM OOAD exam about a month ago, after reading
UML Distilled and
Applying UML and Patterns.
The only mock exam site I used was the IBM "ICE", which I thought was pretty close to what you'd see on the real exam. The ICE database is pretty substantial and you can take the mock exam many times.
I'm not sure what to tell you in terms of ideal preparation time. If you work full-time and only have a couple of hours to study each day, then I'd say a month.
As for the value of
SCJP + UML, again, it depends on your situtation and company. By studying for the UML
test, it helped solidify my OOAD skills, which I was able to put into use recently in designing some of the software systems in my company.
Good luck on your studies!