800mb/s (firewire 800) != 1.5gb/s (sata1)
But its nowhere near as simple as that.
The performance you get from a drive is a combination of the interface real world speed (not the theoretical maximum speed), the drive's head access time rotational latency, and areal density, and the contention for resources (controller bandwidth, CPU cycles, etc)
For starters, FW800 can push up to 80 MB/s if there is nothing else running on the Firewire controller.
3.5" Desktop hard drives have only just broken the 100 MB/s barrier (WD Raptors, Samsung F1 and Western Digital Tornado). So the most a SATA connection can deliver from a single drive is the drive's maximum output -- so 150 MB/s or 300 MB/s are pretty much meaningless distinctions for a single drive, and most hard rives won't deliver more than the FW800 interface in any case.
But wait -- laptop size 2.5" drives have a way lower output, because of simple geometry.
Given the same RPMs (7200 RPM let's say), the amount of data that can be delivered depends on the density of the data on the disk platter, and the length of the track that passes under the heads in 1/7200th of a second. Track length = perimeter -- so a 2.5" drive's outer tracks are at a distinct disadvantage, being 7.8" in length (perimeter) vs. a 3.5" drive's 10.9" length (approximate numbers because the platters aren't exatcly that size)
All else being equal, the laptop sized drive will be 28% lower in bandwidth.
To add insult to injury, a drive's performance decreases as the drive fills up, and the data is written to the inner tracks of the drive. At 90% full, it's typical for a drive's inner track bandwidth to fall to half of the performace it was when empty. So that 104 MB/s performance from the Samsung F1 will drop to 55 MB/s or so when the drive is full --- and a small-capacity 2.5" drive fills up faster than a 500 GB or 750 GB desktop drive... 100 GB on a 120 GB laptop drive is almost full, but it hasn't even caused a 750 drive to break a sweat.
So... that's a long way of saying that a Firewire 800 connection is not the limiting factor in most drive performance, and a 800 vs 1500 comparison with SATA-150 isn't relevant in most cases --- and a FW800 enclosure with a larger, high density (perpendicular recording) 7200 RPM 3.5" drive will likely outperform a 2.5" SATA-150 (or SATA-300) drive, especially a 5400 RPM drive that is 50% full already.