Planning on doing the exact same thing with my Z21. How's your progress so far regarding disassembling?
Edit: Sucessfully replaced my Toshiba 5400 rpm with Intel X-25M G2 SSD. The Z21 is now feeling much quicker to work with. Hibernate/Sleep feels like an instant.
I'll attach a basic guide how I did cause Z21 doesn't seem to have any instructions about HDD replacement around. Of course this guide is only based on my specific system and Sony Vaio VGN-Z21.Step 1
Unscrew all screws (illustrated by red circle in picture 1) on backside. The two orange I don't think are required but I unscrewed them anyway.
Remove keyboard by carefully separate keyboard from the rest by using one or two small screwdrivers in the area shown in picture 2. I found it easiest to start in the bottom left corner (by the DVD), go left and last towards the monitor. There are a couple of small clips holder the pieces together around the whole Kb. Use the other small screwdriver to push those in, and the Kb comes off quite easy.Step 3:
After the Keyboard is loose I choose to just lean it towards the monitor (I actually put some fabric for monitor protection just after photo). This to avoid removing the connector (Arrow in picture 3) to the Kb.Step 4
To remove the old HDD remove screw 1 and 2. I didn't have any on position 3 and 4, but maybe someone else have. In that case those should be removed also. Everything is still kinda tight but by removing all screws it should now be loose enough to remove the metal placeholder 5. You may have to bend a little on the outside plastic next to 5 because there are at least 2 clips and 2 pins holding it in place.
Note which black rubber attached to the HDD corners goes where. They are all in different shapes and it doesn't work just putting them back on different corners.
After 5 have been removed try to wiggle the HDD out by pulling it up and towards where 5 were. Carefully remove the connectors and remember which corner the black rubber corners belong.Step 5
Now it should look like picture 5. Attach your new HDD, use the correct black rubber corners (1-4) and carefully wiggle your SSD into place. Attach the metal (5) previously removed, carefully place the Keyboard on top and carefully push until all clips are secured. Then screw back everything on backside and by HW measurements your done.
Regarding SW. I created a Sony Vaio Recovery disc (2 DVD). By running these and choose Total System Recovery, everything just worked. With this specific SSD I also upgraded the SSD firmware without problems.