I know everyone will suggest getting the biggest screen you can afford, and I agree, but I was recently in a situation where I did have a pretty hard budget.
If you aren’t opposed to reconditioned units, Cabelas.com has the reconditioned 899 SI units for $650 currently. Which is a great price, it’s $350 off retail, which makes it a whopping $850 cheaper than a retail 999. They come with a 90 day warranty(which should get you through the end of the summer should there be any problems). Mine looks brand new out of the box and I can’t notice anything wrong with it.
The added screen size is nice, and some will argue it’s essential, but that’s a huge price difference that I couldn’t ignore.
I have no idea if they are linkable with a 797 though, definitely not the guy to talk to about that :).
Edit: so the pricing would go as follows:
Helix 5″ SI: $499.99
899 SI reconditioned: $649.99
899: $999.99
999: $1499.99 (+ $200 rebate) ~ $1299.99, although it’s a prepaid gift card as rebate.
So that jump from 5″ to 7″ screen is $150, and then the jump from 7″ to 8″ is $650-$850 depending how you look at it.