My old GPU's are 280x and 7950 both have 3G. Will these be sufficient as N increases over time and will I have to be re-tuning cards every time N goes up?
Depends on the coin you're mining. If you set it up to allocate 2680 MB or so (just guessing what it might be), you'll be good at lookup-gap = 2 until N=12. You'll have to fiddle with LG at that point. Anytime N changes though you want to fiddle with your settings - it's not always obvious what the best settings will be. Sometimes running a lower lookup gap and limiting the intensity (using --rawintensity) gives better performance than going with a higher LG and using all the shaders.
For YACoin at N=14, My R7 250 GB card started at 1.6 KH/sec using the OLD yacminer. After I updated the software and figured out how to configure it properly, I found settings that gave me 2.5 KH/sec. I ran with that for a month before experimenting with lowering the LG by 1 and limiting the shaders. I got an extra 8% hashrate by doing this. I will eventually be releasing a spreadsheet or web application for y'all to put your settings into to get the same kinds of results, but it won't be until the algorithms are integrated into YACMiner. I still have one performance metric to come up with a formula for, but I've got quite a bit of data for it now.
I had suspected I might need to upgrade system RAM in my question regarding mining with alternate kernels. I will be upgrading to 8G ASAP, 2G only (which works just fine for scrypt) will hamper my success. Unfortuantely DDR2 which my old Intel DG965RY board uses is WAY more expensive. However moving to a newer DDR3 board would be much more of an investment I am not going to make 
I built my miner using old parts I had on hand ... (other than the GPU's and 1000W PS which I had to purchase.) Still have 2 pci-e slots available to add cards as panic dumpers sell their GPU's on e-bay 
