Another note I forgot to mention...
If you don't want to pay for full gut, an amazing combo is Gut mains/ Isospeed Control Classic crosses. The Isospeed Classic is a multifilament, but very low powered and durable. It will EASILY double the life of your gut mains, if not more. I have used it on multiple occasions as crosses to gut mains, and it outlasts full gut 3 to 1 in overall durability. This is due to the ribbon construction of the string, it resists notching like other multis or syn guts.
The only downside is feel, it greatly lowers the power of your gut, but with lower power comes amazing control. Gives you the control of Gut/poly, with less spin capability and much more arm comfort.
Head Pro Tour 280/630, MP 18x20. Tour Bite 18/Mosquito Bite.
Sampras serve, Agassi backhand and Federer forehand. heh heh