I didn't have the WAG354G but I did switch from a modified DG834GT to a 7800n.
In a word; it was an excellent move, way worth the £110 we spent on it.
It is very stable (It hasn't resynced once by itself apart from when manually resynced or a powercut has happened - touch wood),
Runs passively - I had to fit a fan to my old DG834GT to stop it freezing up and performing poorly; I went through 2 DG834GTs,
Pings are much better and stable with the 7800n,
Wireless is far superior to Wireless G due to having 3 antennae on the 7800n.
The best advantage
for most scenarios: The 7800n has an in-built 4-port Gigabit switch, so if you don't already have a gigabit network (We did) you will benefit in the future if doing large transfers from computer to computer.
The only fault I can see with my 7800n is after you have tweaked the SNR and the modem is restarted for some reason, the slider returns to 0 so unless you have recorded where it was before, you have to use some trial and error to find a good SNR value.
You will probably find you can squeeze more out of your line with the 7800n, but as MrSaffron has asked, we might be able to give you an idea if you post your line stats
You already have an extremely low margin so I'm guessing your lines' SNR varies dramatically during the day, as the DSLAM will try and aim for your set Target's SNR which can be 3, 6, 9 or 12 when resyncing. I'm not sure whether the 7800n will benefit your line as far as speed is concerned.
As for the Ferrari through a 30mph speed limit; It's like that for any router not running at maximum sync but if the road is low quality, the quality of the tires and suspension (How the router handles the line) can really help on a bad road to keep you going as fast as you can go. I no longer force my 7800n to sync at an extremely low margin but I still force it to go faster than it should; If it's stable on this line, it shouldn't have a problem on slower lines.
Here is my 7800n's stats:
SNR Margin(Upstream) 6.0
SNR Margin(Downstream) 1.8
Line Attenuation(Upstream) 9.6
Line Attenuation(Downstream) 16.0
Hope this helps
Connection Speed: 24276 kbps 1211 kbps
Line Attenuation: 16.0 db 6.4 db
Noise Margin: 1.1 db 6.2 db
Telewest (2004-2006): 256Kbps -> 512Kbps
BT (2006 - Present): 8128/448Kbps on 20CN Alcatel DSLAM -> 24276/1211Kbps on 21CN Huawei MSAN
Edited by chris6273 (Mon 11-Feb-13 13:16:00)