Fenix Venom

I've got a macbook, 4GB Ram, 2.4GHZ Processor. I see all these amazing videos of people using SL with such impressive graphics. How do I experience such I quality graphics? I lag like crazy if I drag my graphics preferences up beyond MEDIUM... but even at medium it's so awful compared to HIGH or ULTRA setting. How can I use SL smoothly with HIGH or ULTRA graphics settings? Please don't say buy a PC... because sadly I'm sure that i the inevitable answer :matte-motes-frown: