As a user myself, having a standard notebook with a good screen, a fast CPU, a lot of RAM and a fast HDD will increase performance the most
But most lightweight notebooks don't offer these unfortunately, so you'll have to compromise abit

Perhaps this one?
13"
Dell
Vostro V131, i5-2450M + Intel HD3000@RM1999[1.82kg], 4GB, 500GB(7200rpm), anti-glare, W7HB, colors: silver/red

Use the remaining money to extend warranty to 3 years, and upgrade the RAM to 8GB at least, and also upgrade to an SSD if possible

There are 2 versions of processors:
-Normal processors
- ULV processors (weaker processors to reduce power consumption, hence longer battery lives)

The one I've recommended to you runs on a normal processor, which will be more effective at photo rendering and stuffs. The faster 7200rpm HDD will also be better for faster program and photo load times

i5 are basically dual core processors; whereas i7 are generally quad core processors.
Quad core i7s are more powerful at calculations and multitasking, but generates more heat and consumes more power