the Netgears tend to be tweakable but only if they are broadcom chipset, if by tweakable you mean changing the Noise Margin then yeah, so is the Billion 7800N which even though its popular and good..is also old!
other options are using 1 for modem/LAN and 1 for wifi/LAN - it just means you can combine something like your GT which is tweakable, with wifi off to a router that has great wi-fi (but no modem).
any router that has speeds above 300Mbps (such as 450Mbps) usually requires 3T3R (3 antennas) on the client side, this means your laptop card has to support those speeds AND have 3 antennas internally...most only have 1 or 2 - if on a 150Mbps wifi signal your laptop only syncs to 75Mbps or less then your laptop only has 1 antenna - also, anything 600 and above is a lie!! its all marketing ploy to get you to think you can get 600, 750 or 900Mbps when you can not! a 600Mbps router is 300Mbps on 2.4Ghz and 300Mbps on 5Ghz, you can use one or ther other!! a 900Mbps router is 450 on 2.4Ghz or 450 on 5Ghz, to recap: where it is 300Mbps or less per radio (2.4/5Ghz) you only need 2 antennas - if you expect to get 450Mbps then you need a card that supports those speeds and 3 antennas.
- really as your staying with ADSL its more about range rather than going for higher speeds, get a router that has good reviews for range.
5Ghz is weaker than 2.4Ghz for going through walls etc as its higher frequency, but it is a less congested network, if you have wifi problems and can see loads of your neighbours using all neighbouring channels on inSSIDer
then you could consider 5Ghz but you would need 5Ghz wifi in laptop too.
As for which router is best/good - its just a case of googling reviews I suppose - the best router (without modem) is probably the one Tom bought for his FTTC which is the Asus N66U (Dark Knight) which has just about everything you can expect from a Wireless-N router - 450+450 wi-fi, 1Gbit LAN, USB ports, excellent range, fastest speeds going - but too use this you would need to hook it up to a modem.
if your toying with a N600 Netgear like the DGND 3700 (which I always wanted that one) some people reported issues, cant remember exactly but ive read its poor on ADSL1 connections but good on ADSL2+ - and make sure the version/revision your getting is Broadcom as I think there are two versions now and I dunno if they are both broadcom or what.
Firmware support and feedback for the DGND3700v1 was very poor before it was dropped altogether and superseded by the DGND3700v2 which also has poor firmware support and feedback. The in-built modems on both are very temperamental.
I can't recommend DSL hardware from a company who has a poor track record and even worse just gives up and releases a replacement model.
there is the DGND4000 which seems to get good reviews... its upto 300N on 2.4Ghz and upto 450N on 5Ghz - it uses Broadcom 6361 chipset
the FritzBox's are very popular too (and expensive) they get good reviews