ADSL is a service that runs at a different speed up and down, up to 8 megabits/sec down and 1 megabit/sec up, and is limited to distances of around 18,000 ft towards the high end, and cannot run through various devices that can be placed on the phone line such as a DLC (fiber in the line). IDSL is a service that is based on ISDN technology, runs at a maximum speed of 144 kilobits/sec each way, but can go anywhere ISDN can, at a distance of up to 50,000 ft with the currently used versions, and can run through most DLC.
If gaming is absolutely most important use of the line for you go for the slower SDSL connection because it can give considerably lower pings than ADSL.
If a mix of gaming and download and general usage is more important, you may be happier with ADSL because you can still get low pings, and you can get the faster downloads.
If you have heard even something about DSL, you will have heard about distance. Your phone lines normally terminate at a telco office, usually nearby. This distance, (the length of your line between your location and the telco office), is a very important factor in whether or not you can get DSL, and what speed you can get.
Here are some rules of thumb for distance ranges. Please be aware that especially with non Telco ADSL lines, distance limits for speeds can vary widely from company to company. There are cases where it is policy for "residential" DSL lines not to be offered as far out as functionally identical "business" products!
Less than 5000 feet you will have little trouble getting all speeds of DSL
5000-10600 You may have trouble getting the highest speeds on offer
10600-15000 The danger zone for DSL from national CLECs like Covad and NorthPoint. Speeds on offer are pinned back steadily until they reach 192k for 15,000 feet. If your line is longer than around 15000 feet, they may not accept an order.
15000-18000 In this range, Telco ADSL is normally still available, although it may be restricted to 300-500k speeds
18000-22000 Telco ADSL is not available, although in a few areas, RADSL may be a product you can get. RADSL is speed-variable.
Some smaller DSL specialist CLECs may have solutions for you.
22000-28000 Using less commonly used DSL equipment, it is still possible to use lines of this length.
18000-28000 IDSL is an alternative or possibly the only alternative. IDSL is 144k/sec, about four-six times modem speed.
28000-38000 IDSL is the only alternative
Right now, your choice is probably going to be either SDSL or ADSL. ADSL is generally available more cheaply than SDSL, and usually from your local Telco, although Covad sells a lot of ADSL lines. ADSL is designed for the home user. (RADSL is a slight variation on ADSL, which supposedly offers a longer reach).
If you are going to be operating servers I would suggest SDSL if you can get it, not only because the maximum upstream speed is generally faster than ADSL, but because your DSL provider is more likely to understand your needs, give you a fixed ip and be more responsive to technical problems.
VDSL is DSL delivered over fibre to some lucky lucky people, US West, Gilbert, Phoenix being one example. It runs to 56mbits and will support 4 TVs (loads of channels), phone integrated to the TV (caller ID on the TV screen) and 1meg up and down for PCs.
IDSL can be thought of as modern ISDN. The technology for IDSL is different than DSL, and since it is not as mass-market, it is also more expensive. Your money is not paying for bandwidth, it is paying for equipment and installation that is not as frequently performed, and therefore costs more.
To some extent, also, IDSL prices maybe higher because they can be: IDSL is often the only choice if you are sufficiently far away from your CO.