So you're looking at the:
13-inch: 2.5 GHz
- 2.5GHz dual-core Intel Core i5
- Turbo Boost up to 3.1GHz
- 4GB 1600MHz memory
- 500GB 5400-rpm hard drive
- Intel HD Graphics 4000
- Built-in battery (7 hours)
No, the GPU is soldered to the motherboard and cannot be upgraded. The only 'upgrade' that is available is when you jump to the 15" models, where you can select either a NVIDIA GeForce GT 650M with 1GB of GDDR5 memory or one with 512MB of GDDR5 memory.
Still, unless you do use a lot of heavy GPU graphics apps, the Intel 4000 should work just fine.
Now as to what you can upgrade...
You can upgrade the hard drive - if you need more space, want a faster drive or (my preference) want to install a Solid State Drive (SSD). And you can upgrade the RAM to 16GB (see this Crucial link). Or just 8 GB of you like.
That's what you can upgrade - as your needs change or as your budget allows (but I would make upgrading to at least 8 GB of RAM a must).
Does that help?