These are just my personal experiences with using common hardware to run Mikrotik as a Router (first two) and a Bridge (last)
P4 2.8ghz (533mhz)
512mb Kingston DDR333
INTEL BOXD845GVSRL-2 INTEL 845GV CHIPSET Motherboard
Onboard LAN: Intel 82562ET 10/100Mbps Fast Ethernet
PCI LAN : Intel 82559 Pro 100 PCI Card
Routing between Cox 100mbps full duplex service and 100mbps full duplex lan
25mbps on average uses 7-15% CPU utilization in Mikrotik 2.8.23
Peak RX 4000 p/sec TX 5000 p/sec
Celeron 2.8ghz (533mhz)
512mb Kingston DDR333
ASUS P4P800 SE INTEL 865PE CHIPSET Motherboard
Onboard LAN: Marvell 88E8001-LKJ based LAN controller
PCI LAN : Intel 82559 Pro 100 PCI Card
Routing between Cox 100mbps full duplex service and 100mbps full duplex lan
23mbps on average uses 12-18% CPU utilization in Mikrotik 2.8.23
BRIDGE MODE between nics (firewall basically)
Celeron 2.8ghz (533mhz)
256mb Kingston DDR333
INTEL BOXD845GVSRL-2 INTEL 845GV CHIPSET Motherboard
Onboard LAN: Intel 82562ET 10/100Mbps Fast Ethernet
PCI LAN : INTEL PRO/1000 MT PWLA8391MT DESKTOP ADAPTER
Bridge between Cox 100mbps full duplex service and 100mbps full duplex lan
13mbps on average uses 7% CPU utilization in Mikrotik 2.8.26
RX 2000 p/sec TX 2500 p/sec
All three scenarios used a Transcend 128mb Flash IDE device (TS128MDOM40V) from memorycity.com ($31) as the main storage device with a cdrom drive just for loading the software.