What system specs are you running to try for 7x500? The P5E-VM?
That's pretty amazing if so.
You'll usually need additional FSB & NB voltage, & obviously enough vcore to hit that.
And not all motherboards/CPUs can do 500 MHz.
I could do that with some of my CPUs on my mobo, but not with others.
I wouldn't necessarily be trying for 7x500 1:1.
I'd rather go for 8x438 4:5, though that will be really pushing things with your G.Skill (DDR2-1095).
If your mobo does 5:6, that'd be ideal for 8x438 (3.5 GHz) 5:6 (DDR2-1051).
High TRFC is needed to get that G.Skill much above 500 MHz though, & nearly all Asus boards have a max of 42 unless you run SPD settings (which would likely be fine; just not tweakable obviously).
You can experiment with different combinations & see what works best.
As for how i get my mixed kits to work together?
I just put the pairs in the corresponding channel slots & it works fine.
This mobo is crap for clocking 4 dimms, & is utter garbage for clocking 4 dimms 4:5, so i presently am running 10x360 1:1 (DDR2-720 4-4-4-12) 1.9V
Though the G.Skill is rated 1.8-1.9V & the Mushkin 2.0-2.1V, they are both Powerchips & both will run at 1.8V just fine if so needed.
The Mushkin scales better with voltage; i could hit DDR2-1100 speeds before HCI Memtest started causing BSODs.
The G.Skill could only hit around DDR2-1000, but i have the DDR2-800 kit, & again, not a great mobo for clocking this kinda RAM.