The Sata adapter won't make them go any faster and only some apps would be faster in RAID0 most others would coincide with Venomous' post so I too would suggest you go that route. Pro's by going with the single drive far outweigh the RAID0.
1) 1 older 40 gig HD in RAID0 may result in a quicker Failure rate.
2) 3yr warranty with the 80 gig SE can still be had.
3) Added cost of SATA cable and Adapter for the new 40Gig HD to do RAID0
4) Very little difference in Speed and finally, if you are not familar with RAID0, you may be pulling out your Hair setting up