I got my Pro system (60GB, $300 model) at Best Buy and it has the Jasper board. Find a store that has the boxes out in the open, so you can look at each package (Best Buy had them laying around on the ground
). Look through the serial code cutout and you should be able to see the label. My console has a MFR Date of 2008-11-06 (November 6th 2008). Usually if you can get one made in or after November 2008 you'll get a Jasper. You can identify a Jasper 100% by the power rating. The old 360's (Xenon and Zephyr models) use 16.5A on the 12V input (you can identify this on the back under the hole in the label where it says 12V=16,5A;5V=1A). The Falcons use 14.2A and the new Jaspers use 12.1A. The Jaspers may also come with a 150W power brick, though mine came with the older 175W one (power bricks are forwards but not backwards compatible).
The Jasper boards are still hard to find in Pro systems, but almost all Arcades being sold are now Jasper. Also, the new red Resident Evil 5 Elite packages have Jasper boards according to numerous reports I've read. Just check the MFR Date and don't buy it unless it's November 2008 or newer.
Also beware that not all games support System Link and you'll need one game disc for each Xbox 360 console. Some games may be played locally through Xbox Live. Though GTA IV has no System Link mode, my friends and I hook up 2 consoles to one laptop (to get Internet), connect both to Live, and then play 2 players using Live.
As for something to split the video, I wouldn't count on it. You could theoretically hook your 360 to a computer with an HDTV tuner and then use a program to split the video into 4 monitors, but each would be low quality and it would be a mess to set up. I think you're better off buying another console. If you want a mega LAN party with lots of consoles, you could try buying up a bunch of broken consoles on eBay and doing the X-clamp replacement fix for the RRoD (I did this on a broken console my friend gave me a year ago, still plays fine today though I got a Jasper last month). If you get 4 consoles for $30 and only fix 3 of them, you still can have a good LAN party, add some cheap VGA cables (got one for $2.50 on eBay) for your monitors and some used copies of games and you have an awesome game room.