Hi Cliver, my setup was similar to yours (albeit with ADLS2+), I also prefer to have a dedicated router with separate WAP's and modems etc.
I also had an RT-68U as my main router and used a Billion 7800N followed by a Zyxel VMG8324 in bridge mode as modems.
Personally I had problems using the RT-68U in this way as it would periodically and regularly drop the PPPOE connection for no apparent reason - I replaced it with another central router and it is now just used as a WAP on my network and doing a great job.
I don't have any experience of FTTC connections but tend to stick to Broadcom based devices as I have had by far the best and most stable connections with them on my noisy long line. Many people on here also favour them, the HG612, Billion 8800NL and Zyxel VMG8324/8924 being commonly recommended by people.
If your TalkTalk router is a Huawei HG635
like this one on ebay it might be worth keeping it as I believe it is based on a BCM63168 chipset which is actually considered a good performer and according to user
kitzuser87430 can be used on any connection unfortunately not in bridge mode.
Personally I rate the Zyxel VMG8324, I have just bought a second unit and it can be used on both VDSL and ADSL connections.s
I am sure one of the more knowledgeable peeps will be along shortly to give further advice.
Cheers
Chunks