it kinda depends... if you're on a budget, you'll probably need to stick with crop bodies. FF bodies like the Canon 5D Mk II are pretty spendy, starting at around $2000-$2500 US. as an alternative, you could pick up a used FF body, like i did, for about the price of one of their high-end crop bodies.
i find the biggest advantage of the FF cameras is that, as Tirediron mentioned, the sensors are larger, so the pixels can be larger and the pixel density can be lower, both of which result in less noise, though many of the newer super-high resolution cameras like the Canon 5D2 have crammed so many Mp in that even the FF sensor is getting too crowded.
the best way to figure out what to get, is to decide what you want to use it for. if you shoot mainly landscapes, you might want the FF body for the wider view it gives you, and the superior image quality compared to many crop bodies. if you shoot mostly sports, wildlife, or other shots where longer "reach" is required, a crop body might be better, since it effectively multiplies the focal length of your lenses by 1.5 (Nikon) or 1.6 (Canon) times. that's not to say you can't shoot landscapes or portraits with a crop body, or shoot birds with a FF camera, but there are advantages to both that make them more suitable for some kinds of images than others. that's the biggest reason i have one of each...
the discussion of lenses is good information. make sure that any lens you buy, even for a crop body, will work on a FF camera. there are two reasons for this. one, you won't have to buy new lenses if you someday graduate to a FF camera, and two, on a crop body, the sensor is only "seeing" the center 60% or so of the image circle, which is the sharpest part, so a FF lens on a crop body will often give you great image clarity. as an example, i have a Tamron SP17-35 lens that i got to give my 40D some wide angle capability (before i bought my 5D). on the 40D, that lens is roughly equivalent to a 28-55, a fairly decent landscape range. the 17-35 works very well on the 40D... sharp and clear, corner to corner, with very little distortion. but when i tried that lens on my 5D, it was horrible! the corners were blurry and dark, and the only way to come close to usable results was to stop it down to f/11 or smaller, which is not a viable alternative all the time. fortunately, my 24-135 turned out to be a gem. it's wide enough for most landscape work, and it's sharp and clear edge to edge at all apertures.
anyway, i use my 5D for landscapes, with the SP24-135, and my 40D which works nicely with my 100-400L for wildlife shots. one of the things i've noticed about the FF images is that they seem smoother, silkier and cleaner than the images i get from the crop bodies. that's because of the low pixel density. the 5D was for years - and to some people, still is - the gold standard for DSLR image quality. the new generation of 20+ MP cameras from Nikon and Canon have easily surpassed it in terms of resolution, but the jury's still out on whether they produce better images, or just bigger...
Bookmarks