Hi Ruwini,
You can convert Mbps to Gbps by simply dividing by 1000. That's derived from the definitions of Mega (x 10^6) and Giga (x 10^9). Be careful though... when referring to Bytes (rather than bits) you need to divide by 1024 to get from Megabytes to Gigabytes.
Anyway... As pointed out by dennish, calculating a throughput in packets per second is possible only when a particular packet size is used.
If we take the 65Mpps and look at a "worst case" 64byte frame, you'll need to add to that the headers (14 or 18 bytes, depending upon whether there is a VLAN tag or not) and the inter-packet gap (12 bytes, IIRC). So, each 64 byte frame actually consumes 64+14+12 (90 bytes) or 64+18+12 (94 bytes).
24 GE ports with 1Gbps in and 1Gbps out = 48 Gbps
(48000000000/8)/90 =~ 66Mpps
(48000000000/8)/94 =~ 64Mpps
65Mpps is a reasonable average.
Those are the Layer 2 switching capabilities of a 24 port switch based on the available capacity on the ports. The actual switching capacity of the PFE is much higher than that (declared at 88 Gbps for the 24 port switch). So, the PFE is capable of significantly more throughput than the interfaces can offer to it.
Rgds,
Guy