need some help to set UP power mac G5

hello experts,
i am totally new to mac, what i know that i read on net or forum like yours
one of my friend gave me power mac G5 2.7(DP) /pci-x/4 gb ram
he dosen't have any installtion and i put new sata HDD , i download (window based computer)osx 10.5 leapard.dmg (6.9 GB)
my problem how to install it ?? is there any other mac osx upper than 10.5 can i install ?? ( 10.7 lion or so )
- DVD support only 4.2 GB , (i don't know if my drive support duel layer or not)
- is it possible to boot from USB external stick ??? bcoz it's ppc processer !!
- another main issue is wifi , back of my tower i have antenna connection ( i don't know yet is there airport card or not ??until i dismental it)
i assume there is nothing , can i use any USB wifi adaptor with this G5 or not ???

Thanks in advance , help appriciated