I think realistically we will see few if any dual layer upgrades for existing hardware - not least for commercial reasons. It seems more likely now that dual layer will almost certainly be done using new chipsets optimised for dual layer use..
My understanding is that Philips have announced a forthcoming dual layer chipset for dual layer DVD+R. Meanwhile, it seems that the consensus is that nothing apart from 8x DVD writers are likely to have the necessary laser power for dual layer writing. There is still debate as to whether the 106 drive Pioneer used in a public dual layer writing demonstration (on test media) had hardware modifications or not - and even if it didn't, the chances of an already discontinued drive getting a dual layer firmware might well be remote.
I think the reality of the situation is that there will be dual layer products emerging later this year. Dual layer DVD+R seems as if it will be here first - probably in about three months' time, though that could slip. I believe the first products are likely to write the first layer at 8x and the second at 2.4x. It might well be that you have to write equal amounts on both layers, so a dual layer write will be significantly slower than two single layer discs on modern hardware.
Dual layer DVD-R seems a little more distant.
I wouldn't be surprised if the first dual layer products are fairly rapidly eclipsed by later generations, and the first dual layer media is likely to be very expensive. Whilst I have some sympathy with the idea of holding on if you have no particular rush to buy now in the hope that the first dual layer writers will be just as good as the current single layer writers and not have that much of a price premium over them, the truth is that dual layer writing is likely to be slow, require expensive media, and might go through several generations fairly quickly.
I was considering holding on, but eventually realised that a good single layer writer isn't that expensive now - and as I could do with DVD writing capabilities now, I could always upgrade drive again in another year when the dual layer products are more mature and media costs are likely to be lower. I'm not really interested in writing DVD video anyway - my requirements are for data discs, and >4GB is a huge step up from 700MB on CD-R.
So, I bought a NEC ND-2500A, which I'm very happy with. It's a huge improvement over the CD-RW I had before, and cost me 88 pounds in the UK (the price is even lower now, I believe).