Before you pay for any plan, find out exactly how far away you are from the central office. You could spend the extra money for the higher bandwidth and Cox will happily sell it to you, but if you're beyond a certain distance from the CO, you'll never hit those speeds, though you'll be paying for them. Lines speeds for ADSL (asynchronous DSL, meaning download > than upload) are always qualified with an 'up to.' 'Up to 4 Megs!' There's a lot of variance between 128K and 4Megs and the odds are, unless you're sitting on top of the CO, you'll never come close to that 'up to' speed. Ask the provider what speeds are gauranteed.
As for hardware, look on eBay. You'll find whatever modem Cox requires on eBay for way less than $99. A router is optimal, though, since you're looking at ADSL, for security reasons.
Have a look at
www.dslreports.com. This is a GREAT resource, not only for looking up ISP's and reading reviews on them, but also for tweaking your connection and verifying the lines speeds you're getting - look in the Tools section.
Cheers,
A.E.M.