Mpps is million packets per second.
". . . can load 2Gig throughput"
That depends on the nature of your traffic and your configuration. I.e. maybe, maybe not.
Georg provides the old Cisco router performance sheet. If you read the top, it tells the bandwidth is calculated for 64 byte sized packets (Ethernet minimum). (Also note, this sheet's bandwidth calculation doesn't take into account L2 overhead - i.e. it doesn't account for actual bandwidth on the wire.) Usually the device's bandwidth capacity goes up (a lot) as packet size increases. (Also note, usually device's PPS capacity decreases as packet size increases.) Also usually the bandwidth capacity goes down when you start "doing things" to the traffic (e.g. passing it through an ACL).
Also remember, the PPS and/or bandwidth may need to account for duplex traffic. For example, if you have a gig Ethernet interface, using full duplex, you would need to support 2 gig of bandwidth. (Also BTW, you need about 1.488 Mpps for a gig of Ethernet bandwidth, at minimum Ethernet/IP packet sizes.)
From personal experience, using that old Cisco router performance sheet , I've found you often want to "max size" a Cisco router at about 1/4 the bandwidth capacity listed when dealing with full duplex traffic. I.e. for a G2, it's good/safe for about four to six 100 FE ports (running full duplex).
Also keep in mind, with gig ports in an VXR chassis, you'll also begin to hit the chassis bandwidth limits too. I recall (?) the 3 (?) buses are gig PCI.
