So here’s my experience on my Dell D420 running Vista Business RTM. System specs are 1.2ghz dual core Intel U2500 cpu, 1.5GB RAM, 224MB Video (shared), 60 gig HDD (36 dedicated for Vista).  Vista Performance specs are here 

1.  Ordered a 4GB compact flash card.  Slapped it in a PCMCIA adapter and into the Type 2 slot.  Saw it as removable media, but couldn’t use it for ReadyBoost. Rats.
2. Took my 1GB Sandisk Ultra SD card and popped it into the SD slot. 
3. Vista recommended that I only needed 870mb for ReadyBoost, so the 4GB was overkill anyhow – kind of a pisser.
4. ReadyBoost file created after about 3 minutes.
5. Started fooling around – seems much slower.
6. Figured I need a reboot, so I did.
7. System took 10 minutes to boot up to CTRL_ALT_DEL.
8. Logging in took forever as well.
9. Outlook 2007 opened after 5-6 minutes – normally about 20 seconds as I have a 4gb OST file (I know.)
10. Had all sort of issues with Outlook updating folders (running cached exchange mode).  Messages sat in the Outbox for 45 minutes.
11. Tried to use the box the better part of 2 days.
12. Took the ReadyBoost option off of the SD card and rebooted.
13. System still slow.
14. Removed the card. 
15. System still slow.
16. Rebooted again
17. System back to normal.

 Ugh, what an ordeal.  Definitely needs some work in my book as this is a lower end business laptop that has a dedicated PCMCIA and SD slot and neither device was effective at all for ReadyBoost.