This is a case where new is preferred over used for the reasons mentioned above. The older systems had a "high" chance for an overheating issue referred to as the RROD (red ring of death). The system would heat up and melt some of the solder, causing the system to fail. Microsoft did finally admit that it was their problem and offered a 3 year replacement vs the standard 1 year, and refunded people who paid for the repair (if it was within the 3 years). Problem is, this often only applied to the first purchaser. If you buy used from Gamestop or somewhere else, you don't know if the system is newer or older, if it is older it may develop the overheating issues. Since it is out of warranty, you would then be out an additional $100 for the repair. You could get the Gamestop replacement plan, but that only covers you for a year.
Yes, Gamestop systems are tested and do work, but they do not test the online gameplay. There are times when people cheat/hack and get their SYSTEM banned from online gameplay. These folks often trade the system in to Gamestop (since it works when they test it they WILL take it as a trade) and get a new system. This then leaves people who buy the used system to deal with the problem of the system being banned from Xbox Live. Yes, Gamestop SHOULD take it back for a replacement, but it is not a sure thing.
The Gold membership is required for online play/netflix. As previously stated, this can be purchased at a discount (normal price now is $60/1 year, I usually never pay more than $35). It involves some searching and purchasing before the subscription is up, but worth the time. Games (new) can often be found at any B&M shops with some sort of giftcard attached to the purchase (K-mart and
Amazon often run a $10-$20 GC with the more popular games, and Amazon usually attaches free standard shipping as well). Buy the first game and then all future games are already discounted, and often you will get the GC back (kind of a lather, rinse, repeat process), if you watch the sales that is. Also, most new games are often discounted as soon as 2 weeks after release ($40 vs $60 a few weeks earlier). This can be a way to get great games at a great price as well, but you sacrifice a few weeks for the savings. TRU and
Best Buy will usually run a buy 2 get 1 free promo at least once a year.
Back to the new system vs old to close with. Most retailers will, from time to time, run a GC with purchase of a system. This would work for all systems in stock (usually), the special and holiday bundles are the best deal since they also come with games. For example, this week Target has the Gears of War 3 limited edition system for $400 with a $40 GC. It comes with GOW3 and an extra controller, a $120 added bonus. The $400 is the same price as the standard system would run (for the 250 GB hard drive), so the game and extra controller are of no additional cost. They should have a new holiday bundle coming out around thanksgiving, which will probably include 2 game and a smaller hard drive (120 GB) for $300, but no news as of yet.
You are better off getting a bundle than a standalone system, new vs used. This way you get full warranty (3 years) and some added value (games, extra controller). Also, I would stay away from the "arcade" version, as this does not come with a hard drive. It does have 4 GB of internal storage, but this will fill quickly if anything is downloaded. Watch for free GC sales, and a really great deal can be had. PM me if you have further questions, or need something cleared up.