Dual layer is around 8.5GB.
NEC have announced the ND-2510A, which isn't yet available. It adds DVD+R9 (DVD+R Dual Layer) support to the ND-2500A. As the FCC filing for approval of the ND-2510A says that it's the same hardware as the ND-2100A (the DVD+R only version of the ND-2500A sold to some companies like Dell), some have got excited at the possibility of a (likely unofficial) dual layer upgrade being possible.
However - I wouldn't get too excited. Early DVD+R9 burning will probably be at 2.4x rather than any faster. Even when drives are available, the media is likely to be expensive and in very limited supply - it seems that many media producers won't have production fully ramped up until the third or fourth quarter. By then, I wouldn't be surprised if there's a second generation of dual layer writers available with maybe 4x dual layer writing (a huge speed saving).
DVD-R dual layer is also quite possibly coming - though probably not until around the end of the year.
It is, of course, possible that drives sold as ND-2500A will lack the necessary factory calibration data to write dual layer successfully (the cost of a few dual layer coasters could go well towards the cost of a drive with dual layer support from the factory). It's also possible that there'll be subtle changes in the optical side of the drive, making a dual layer upgrade impossible (though I'm not sure whether this would be within the latitude allowed by the FCC filing).
I hope that helps to put the issues into perspective. Whilst you might be able to upgrade the drive to dual layer DVD+R operation, the capability is likely to be first generation, possibly soon superseded by a second generation of dual layer drives, and with seemingly very limited availability for what will be expensive media, you may find that if you're serious about dual layer writing, you'll be wanting a new drive in another 6-12 months.