Towguy and jeepit, that's the million dollar question, LOL...and I'm not nearly as fluent in turbo-speak as many others are. The trick is going as big as possible without loosing spool and drivability, and be towable. A good and fairly popular route is the Switzer S300 family, due to their decent performance and durable turbine shafts. The out-of-the-box ones are in the $800 range but you don't want one of those, would be better off spending more for the modified variety, which is more like $1500-1700+. Vendors that sell them for example are BD Power, Source Automotive, Diesel Power Products, Industrial Injection, High Tech Turbo, etc. I'd suggest a 62/65/12 (or if you want a little more breathing on the exhaust side do a 62/65/13 stainless, but more bucks). Either is a good step above what you have now, which is a 57mm compressor I believe, and 12cm exhaust housing compared to the tiny 9cm on yours.
But when you get into the $1700 range there is another turbo that tops the S300 family, if you are willing to plumb in coolant lines, since I'm talking about a water-cooled dual ball bearing turbo. That's what I have, I really like it. It spools faster than others of comparable size since the shaft is lighter, and less drag because of the bearing. I suggest the Stage 2 which is the Garrett 3782R (also known as Power Max Stage 2) from Turbo Re-Source, it is even larger than the turbos I mention above but will spool better, fairly close to stock. With a stage 2 you can run a bigger injector and Smarty and be able to use the power more readily. I have the Garrett 3788R, the Stage 3, but they require you to run it with a cam (otherwise it will surge). I don't have a cam, but mine doesn't surge since I have an equal-length tubular header. It doesn't spool quite as fast a stock but it's very drivable. Here's a video of my truck running around at 5300 ft elevation
http://youtu.be/E4anPv_S1i4 Another showing the turbo & header, pardon my lousy narration LOL
http://youtu.be/1YB7VNvv9dk
If you go dual BB only go to Turbo Re-Source to get it, since they do mods to them other vendors don't. You will need a new HX40 downpipe to go with it to tie into your exhaust. Talk to Tom Spictig, use the online contact form and you should hear back from him
http://www.turboresource.com/