The problem of low GPU usage, more often than not, can be reduced to a small handful of causes. The first: CPU bottleneck, which becomes painfully apparent in multiple GPU configurations with "antiquated" chips. Sometimes misinterpreted as the aformentioned is poor driver optimizations (probably manifested as outdated catalyst/forceware updates), and as a case in point the initial Eyefinity drivers running off multiple 5870 cards. It was common to have GPUs connected to top-end Nehalems seeing teens to twenties performance though everyone knew these performance scenarios were GPU limited, not vice versa. Finally, PCI-E bandwidth saturation can result in negative scaling type results (i.e. 3 cards functioning at low percentages), though I've personally only seen this on ***** P55 motherboards with a third PCI-E x16 tacked onto the platform control hub's generation 1.0 x4 lanes (which also is shared by the chipset sata II, ethernet, et al.)
The reason you're getting 100 percent usage in certain benchmarks is specific to the way benchmarks are coded. It doesn't mean that, because you've seen near maximized GPU utilization in one evaluation (benchmark whatever), since other software demonstrates far poorer performance comparatively, the performance deficit is because of an intrinsic flaw in AMD's drivers, crossfire scaling, and so forth.
In ATI's VLIW-5 architecture, the so-called t-unit was devised and implemented when DirectX 9 & vertex shaders kept the widest SP occupied. Transcendental operations (accounting for a dramatically higher proportion of overall code when the architecture was fresh) have become less often utilized from year to year, such that essentially 1/5 of the units are, on average, sitting idle.
The point being, benchmark software like Vantage, 3dmark 06 and whatever optimize software to, in a sense, achieve a higher performance result than ever would happen in the context of..well, reality, by utilizing shaders that more often than not are remaining idle in games.
Upgrade to an i7. It'll resolve all the problems noted here, & the shift in performance is night & day difference