> Will filling this out impact the negotiations at all?
Highly. Filling out that form eliminates negotiation completely. You're giving up your informational advantage and letting them set your salary.
> I didn't really have a plan other than asking for my current salary + 20%
Quite frankly, you're worth more than that.
Here's a few questions that I advocate my friends ask themselves during their job searches.
1. How much do I need to make?
What's your budget? How much do you need to make month to month to survive? Okay, that's the base line. You can't earn less than that or else, by definition, you aren't earning enough to get by.
What are your savings goals? How much do you want to save month to month? Add that to the baseline.
How much more do you want to make? How much mad money do you want each month that you can consciously spend on whatever you want?
Add those together. That's a pretty good indicator of how much you want to make to be happy. Now, it may be that this number (x) is less than what you're already making (y). If so, congratulations, you're in a great position, but that doesn't mean you don't deserve more money.
2. What's industry compensation?
For the job title you're interviewing for, what's industry compensation like nationwide? How about locally?
Use salary search sites, informational interviews, and beers with people working in your industry to figure out about how much these types of positions pay. It may end up being more than your x + 20% estimate.
3. What's company compensation like?
Do you know anyone who works at the company? It's google, so you probably do. Can you find out what someone starting at this title makes to start? What's the work load like? What are the other perks?
4. Aside from cash, what do you want?
Google has nice swag, but is there anything you need that you can ask for outside of cash?
Maybe you want more vacation time. Maybe you want to work from home 1 week each month. Ask yourself 'What would make this job awesome to me.' Then ask for that.
5. Additional Resources
Here's two articles that can help with this decision.
The first is 'The Business' by rands: http://ka1.us/dgX6
It's a great overview of figuring out how much you're worth and getting them to agree.
The second is an article I wrote at the start of March about negotiation: http://ka1.us/fxki
My article goes into a bit more depth on identifying how much you're worth and asking for it.
I hope these help! Congratulations on the job offer!