my advice would be to stick with i5/4gb ram and opt for 256GB SSD.
You're getting twice the SSD speed from 128GB, and the storage space is going to be the most noticeable for you and your usage.
For your purposes, 4GB ram and i5 and way good enough. It's just this internet myth that 4GB isn't enough to run a computer in 2013. That's ridiculous.
Mavericks is being released as well, it will free up to half your memory usage by compression.
Apple seriously isn't going to sell 4GB RAM in their machines in stores if it can't handle the weight.
The thing is 8GB Ram is good if you want to run windows/linux on parallels.. or even both.
but just to run OS X, especially Mavericks 4GB is plenty. Also, people that do music/video/photo editing and so on sometimes prefer more RAM. not the case here.
For i7 - you won't notice any difference for your stated usage.
tl;dr- If in budget (just $50 on top of the i7 upgrade you're considering) - get 256GB SSD. Right now you think you will manage with 128GB, but it will be a real pain in 6 months or a year.
Ask anyone with experience in using a 128GB machine. And as said, 256GB = 2x speed of 128GB.
Good luck!
Edit: Despite the myth being that newer OS means more RAM needed, Mavericks actually works with just 2GB RAM due to memory compression:
http://appleinsider.com/articles/13...avericks-aims-to-free-ram-extend-battery-life
Good Read