ADSL = Asynchronous
SDSL = Synchronous
As in the upload speed being the same or lower than the download speed depending on which one you have.
If it were me I'd go for the 768k package with a single static IP depending on price. If the static IP is too expensive (40 dollars more is too much, under that and IMHO it's worth it) the dynamic option isn't bad either really, since it's just DHCP and not PPPoE or PPPoA. You won't need your current router, or you might, depending on how you go about it

. If you download alot from Usenet or other such stuff you might benefit from the higher download speeds, but getting over 100k/sec downloads from websites is pretty rare anyways, so 768k would probably be the best deal. Your upload will likely be capped at 128kbps, 256kbps, or 384kbps.
If they'll cut you a good deal on one of those Cisco 678s, or if you can find a used one from someone else around there who had DSL and cancelled it, that'd be great since it *is* a router, and you could therefore sell your current router and recoup some of the cost involved in this switch in services.
If you can't find the 678 cheaply enough, you could get a regular ole modem to plug in in front of your current router, but you'll have to find someone with Alltel that knows the specs on their CO setup. Seeing as they're dealing with Cisco DSL routers the line protocol they're using is probably CAP, but the 678 can support G.Lite and DMT as well with a firmware swap, so be sure before looking for another modem. Call Alltel and ask which of those three they use for DSL and let them transfer you around until you get someone who actually knows

. If it's CAP, your choices are pretty limited since CAP is proprietary to Westell chipset devices. If it's DMT you have more options since this is a more open standard that many people make bridges/routers for. G.Lite is all but dead, highly unlikely they're still using it (although it's possible I guess).
Bottom line, find out what you can get and what they're using on their end. Once you know what you can use, get the gear. Those little Cisco DSL routers are fine and work great, but they're not as user friendly as your 4 port router. The Cisco web interface for their routers is limited, you'll have to set up most things via the command line. The Ciscos far outpace the common aftermarket DSL/Cable routers in functionality, but you'll have to get used to working with stuff from basically a Unix-type shell.
AOL/TimeWarner merged, giving AOL the ability to move all TimeWarner ISP customers to AOL's network, it's just now finally happening.