Dear Mates,

What is better, to connect Squid Box to Mikrotik System through a cross-over cable with 100Mbps NICs on both OR to connect them using 1Gbps NICs, or To connect both of them to a 100Mbps or 1Gbps Switch?

Knowing that the WAN interface of Squid is connected to a 10Mbps of bandwidth..

Crossover, to avoid switching delays.
Whether to use 100MBit or 1 GBit interfaces, depends upon load and processing power/disk speed of squid-box. Because of the rather small difference in price, I would opt for the 1GBit interfaces, unless 100MBit interfaces already available, thus "for free".

Crossover, to avoid switching delays.

also, 'RAM Only' cache in Sqiud - to avoid disk accessing delays =)

in general, if you have gigabit switch, then use 1 Gbps NICs. if no - 100 Mbps is quite enough for such setup