UDP Buffer Sizes in RAC

Hi
on our 2 node 10g R2 RAC database, we are seeing high waits for "gc cr multi block request "
Besides, full table scans, other suspected candidate is the UDP settings in the sysctl.conf file

we have the following settings in place

more /etc/sysctl.conf |grep net.core
net.core.rmem_default = 262144
net.core.rmem_max = 4194304
net.core.wmem_default = 262144
net.core.wmem_max = 262144


How do we know if these settings are OK or if should bump these up? If we should bump these up, what values we should use?

thanks