Agree with above, and I will state unequivocally that if it was my choice, I'd stick with the "old" L3 switches, unless there are some compelling features of IOS that you need to employ. One such feature is NetFlow. This would give you a tremendous amount of information about the traffic flowing through the router that you will never get from the switch. Maybe some other features like GLBP, SLA, PBR, NAT, OSPF, BGP, or something else where a router excels.
From a pure switching/routing perspective you will potentially lose some performance. But, the 2851 does have 2 Gigabit interfaces (not 10/100 as stated above) that can be sub-if'd with vlan tags and trunked to the switches to provide the necessary L3 between the L2 VLAN's, so it won't be that much of a loss unless you are processing a HUGE amount of data. Perhaps in a Medical application where you are pushing huge graphics images of X-Ray or MRI studies, or 3D PET scan data. Or in a graphics intensive shop like Pixar studios doing huge amounts of animations...
Normal day-day office network traffic will never see that kind of utilization or ever notice the difference.
All that said, the "brand new" 2851 is being phased out for the "brand-spanking-new" 2900 series that will run circles around both the 2851 and the 3550.. The 2951 has 3x 1GB interfaces with one of them a SFP port.
http://www.cisco.com/en/US/prod/collateral/routers/ps10537/data_sheet_c78_553896.html
