Anupa,
Couple of points!,
1. In oracle you can store blobs as "out of line". this will make the blobs getting stored outside of the table and hence in the event of table scanning, ti will not scan the blobs !
2. Second point is on cashing; we are using a Oracle( previously tangosol ) product called Coherence for distributed caching !. take a look at
www.tangosol.com. We are using tangosol for similar size storage and our response time is around 2 milliseconds.
Shoot me a mail if you need more info on any of the above
Thanks
raees