"If only Servers with 1Gig interfaces will be connected to the 4948 switch then is there any performance gain while using 4948-10GE over 4948? or there will be no benefit? "
The benefit of the -10GE is the dual 10 gig ports. Both switches offer full wire speed for all ports.
"Does 4948 gives wire speed for typical ethernet packet size or wire speed is achievable only for smaller packet size? "
Should be wire speed for any packet size.
"If 20 servers are connected to 4948 and 10 servers are transferring data to the remaining 10 servers then is it safe to believe that all each server pair will get 2Gpbs full duplex?"
Yes. Also, if you had two server connect to 10 gig, or ten one gig servers with one 10 gig, should support wire speed.
PS:
BTW, the 3750-E may offer similar performance and can be stacked.