Well, lowering the cg will certainly help the handling... The main reason I like a solid frame is that I want the suspension to do the work, and not have the frame flex and twist... With a flexy-twisty frame, you will always have door gap issues and a bunch of rattles and squeaks that get rather annoying...

X-bracing is good, if it can be done in such a manner that it doesn't interfere with the exhaust and other componets under the car... Some boxing plates with good solid crossmembers welded in and a few gussets to support the crossmembers are a lot less hassle to install and easier to build the rest of the chassis around as opposed to an X-brace....