Yes, I am now shooting with both an E-1 and an E-300, and before I go any further - I am in the Olympus Visionary program and am given equipment... That said - the E-1 is definitely the better built, better featured camera. In fact, I don't think there's any question that it is as well built, and ergonomically superior, to the top Canon and Nikon cameras costing 2 to almost four times as much. Yes, the 5 mgp is limiting, particularly in terms of cropping. On the other hand, as I've said before, 5mgp can produce large, gorgeous images.
The E-1 is definitely noisier at 800 and up than the current Canon generation. But the noise problem was much reduced with one of the firmware updates, even though it wasn't part of the official list of fixes. I routinely shoot at 800, and don't have any problems doing so. If I have to go to 1600, I can. Let's not forget that 1600 film is "noisy," and so, for that matter, is tri-x at 800 - which is my film and speed of choice.
The E-300 is clearly the competition for the Digital Rebel, etc. It's well built for the money - has a solid feel to it, but is certainly not an E-1; but then it's not intended to be. Despite, or perhaps because of, the higher pixel number, the E-300 is definitely noisier at 800 and 1600 than the E-1, but the 800 is okay. At this point, I would kill to see two E-300 features ported over to the next iteration of the E-1 - the black and white mode, which is not only the best I've seen in any digital camera; it is superior to what I get shooting in color and using the FredMiranda plug-in in PS. It really IS black and white, and if you shoot raw with JPG, you get that black and white and a color "neg." The second 'feature' is the autofocus on the E-300, which is improved over the E-1. The E-1 autofocus is good and fast, but there are some problems with the 50 f2 macro, and some hesitancy in low light with the 50-200. With the E-300, the 50 f2 is a dream - no focusing delay at all; it's just 'bang on.' And the 50-200 focuses faster.
If you're looking for a pro camera, the E-1 is it. The only question would be do you buy now, or wait for the next iteration, which is said to be reasonably fast approaching. But if you're looking to save money, and the 8mgp is important to you, go for the E-300.