" What an interesting mix of results! Let's begin our inference of the results by saying that for the Radeon HD 5870, the PCI-Express 2.0 x16 is the broadway it can fit all its four wheels on, and try some road stunts, while it's at it. PCI-Express 2.0 x8 performance, which is perhaps the most crucial set of figures in this review, holds relevance to most people looking to pair two of these cards on mid-range motherboards or one of these cards on an x16 1.x motherboard. It holds even more relevance to users and potential-users of most socket LGA-1156 motherboards, as this is where 16 lanes from the processor's on-die PCI-E switch are split into two 8 lane links. Surprising as it seems, the Radeon HD 5870 is comfortable, with a mere 2% performance drop overall. PCI-Express 2.0 x4 is where the Radeon HD 5870's discomfort is slightly notable, with a 5% drop, and even more surprisingly, on PCI-Express 2.0 x1, big as it seems, the performance drop is "only" 25% overall. Considering that you rob the card most of its data transfer potential, leaving only a 1/16th of the optimum bandwidth, it is still impressive that it can deliver 75% of its performance. "
He then goes onto say this, which is still of consideration: "Different applications respond differently to the drop in interface bandwidth, and hence you could do with a closer look at the results for each application. Games with lighter texture, shader, and instruction data don't particularly need all 16 lanes, and evidently, in games such as Quake 4, you're able see the accelerator comfortable with even PCI-Express 2.0 x4. Video memory-intensive games will show bigger performance margins. Besides applications, the other important factor is the resolution at which they are being run. At higher resolutions, it pays to have higher interface bandwidth, as it's usually high-resolution textures the GPU is dealing with. Although small, the gaps widen with increase in resolution. However, it is important to realize that even at 1024x768 some applications will see serious differences in performance caused by PCI-Express bandwidth."
So it's dependent on the OP's resolution and types of games he's going to be playing. So if he's gonna be playing Quake 4 as the article writer described, at a resolution of, say, 768p then no the x4 slot will serve him fine. If he's playing BC2 on considerable settings at 1080 or above, it may be worth securing a x8 x8 capable motherboard before thinking of a multi-GPU setup.