Ssd
The OWC SSD is a notable upgrade. The speed you will get will surprise you. What might also surprise you is how easy it is to work on the Air.
I was having some issues with my late '08 air, and thinking it was the HD, I got a 60 GB SSD from OWC. I didn't want to spend a whole lot more on a suspicion hence the me buying the smallest one. The machine became a new again. 10 second boot to start. The actual slowdown however, was in fact due to a bad logic board which finally crapped out a few weeks later: the SSD's speed made the real problem of a bad board obvious with me still having issues with stuck booting and odd video behavior. Found a used one on EBAY for $200. So much for not wanting to spend a lot on a 5 yo machine. With that being said, this model 2,1 is so much quicker with the SSD and assuming you have confidence in the other hardware, it is a SERIOUS improvement. I can even run XP (rarely when I need a program that only runs in win) with 768 MB RAM in a VM with the 2 GB RAM.
Granted. It is probably swapping quite a bit, and the SSD won't last forever because of this, but $1.60 GB isn't bad for increasing the usability of the machine. Keep in mind the OWC SSD seems to be the only one that goes in this model and it isn't a perfect fit: the original HD mount needs to come out. Apple might make one, but the early ones are noted to be fairly slow.
OR, like mgipe notes, get the entire upgrade kit
. I actually got a 5,2 with the i5 and 8 GB RAM. The 2,1 is my travel and work machine. If I were to drop it or have it stolen, I'd still be annoyed but would easily get over it.