I'm not in the market for a 2GB so I am exactly up to speed when it comes to good deals (as you can probably tell; last time I checked 2GB Pro Duos were still in the neighborhood of $400 or so).
There's a 99.99% chance that it will work with your PSP. At this point, there are only two manufacturers that offer memory sticks: Sony and Sandisk and they share the same production line (meaning: the only difference is branding).
2GB has been verified to work with the PSP. As long as you're willing to shell out that kind of cash you're good to go.
Regarding the speed thing: all duos are only guaranteed to work according to specification and that's it. The 2GBs may be a bit slower but they're still within spec (it's a branding requirement and with Sony & Sandisk you don't have anything to worry about).
Just make sure you really need the 2GB. Cause you sure are gonna pay a premium for it. (Hell, 1GB is overpriced right now. 512MB sounds about right but you'd better not compare MS MSRPs to SD MSRPs. 2GB is crazy.).