They all are HDMI now.. and all should be 'jaspers'. Thats the latest chipest/motherboard revision that runs at half the temperature and uses half the wattage of the original 360's. They are much quiet, cooler running and reliable than the older stuff.
Few things you might want to be updated on:
1) Harddrives are no longer an issue. Latest Spring update just made it possible to use generic USB thumbdrives to store games, saves, DLC, demos, movies, marketplace stuff, etc on.. so buying an Arcade is perfectly fine.
2) Arcades have storage built on the board (I believe 512mb) so a harddrive or USB thumbdrive isnt necesssary for game saves. You can play games, online, out the box on the Arcade.. with no additional memory necessary.
3) If you have a Harddrive or USB thumbdrive you can install a game to that.. and you no longer need to play the game off the disc. Makes the 360 run much quieter & cooler. The game disc still has to be in the tray, even though its not being used.
4) Check for Dell.com coupons. Arcades from Dell come as cheap as $160 shipped (no tax). Also check sites like fat-wallet and slick-deals for good deals on Elites or Bundles.
5) All 360's have a 3 year RROD warranty. If your system is less than 3 years old.. send it in, get it fixed free. Even if you dont want it back, get it fixed, get it back, sell it on Ebay and put some dollars in your pocket.
6) There is rumored to a Xbox Slim coming later this year. Expected to be announced after E3 gaming expo in June. Look at the PS3 slim and imagine the same concept for the 360.