M-audio Omni Studio, Has anyone already use it ? Options

[freaky]

hi everyone!

I have a G4 400mhz , and i'm about to buy an audio interface to do recording with logic audio:
I'm don't have big money so i'm looking to thing like the delta 66.
Since i already have a mackie mixer i'm wondering if i really need the Omni studio preamps.
I like to hear about the quality of those, are they better than those from the mackie mixer?
Or would i get better result by going thru the Mackie and then go the computer by the Line input?(in this case i think i don't need the Omni and i can just get the delta 66)
Since i really don't have a lot of money, but really want to have the best sound possible , all that is a big issue for me!
So i will be really greatfull for any advice on this!! (especially from someone who already use this configuration)