I spend some time figuring this out as my first custom target so I figured I would post it.
This graphs the available memory from Cisco devices that use IOS. This however doesn't work against a PIX which I am still trying to figure out it's OID.
# Monitor the Available Memory
Target[10.10.254.254_MEM]: 1.3.6.1.4.1.9.9.48.1.1.1.6.0&1.3.6.1.4.1.9.9.48.1.1.1.6.0:datanet@10.10.254.254
MaxBytes[10.10.254.254_MEM]: 59392000
UnScaled[10.10.254.254_MEM]: dwm
Options[10.10.254.254_MEM]: gauge noo nopercent
Title[10.10.254.254_MEM]: Available DRAM
PageTop[10.10.254.254_MEM]: <H1>Available Memory</H1>
routers.cgi*Mode[10.10.254.254_MEM]: mem
routers.cgi*Icon[10.10.254.254_MEM]: chip-sm.gif
Background[10.10.254.254_MEM]: #ffffff
ShortLegend[10.10.254.254_MEM]: Bytes
YLegend[10.10.254.254_MEM]: Free DRAM (MBytes)
Legend1[10.10.254.254_MEM]: Free DRAM
LegendI[10.10.254.254_MEM]: Free DRAM:
routers.cgi*MBLegend[10.10.254.254_MEM]: Installed DRAM Total