Since the post is tagged low-light, I assume you are primarily asking for low-light capabilities.
There are three aspects of low-light sensitivity:
- Lens aperture: An f/1.4 lens will give almost 3 stops more light than an f/3.5 kit lens. (At the cost of a very shallow DOF, and some vignetting and distortion when used at the largest aperture.)
- Sensor size: Larger sensors can collect more light. Your Olympus has a four-thirds sensor. DxOMark tests sensors (see the "Sports (Low-light ISO)" tab). By the metric DxOMark is using, compared to your current camera you can gain about 1 stop of low-light performance from switching to the best four-thirds camera, 1.5 stop from the best crop sensor camera, or close to three stops from the best full frame cameras. (By using a correspondingly higher ISO when shooting.)
- Sensor generation: Newer sensor generations generally have better low-light capabilities than older generations, and different brands/models may use sensors that are a generation or two better or worse than the rest. (E.g. the DxOMark tests show a 1 stop difference in high-ISO capability between your Olympus E510 and the Olympus OM-D E-M5 - both Olympus, both Four Third cameras, but there's five years between the two models.)
So all told, you can get close to 6 stops better low-light performance by picking e.g. Nikon D600 and an f/1.4 lens, if you're comfortable with the dramatic increase in bulk, weight and price compared to what you have today.
For actual brands and models, you're better off reading reviews, which also cover other aspects like image quality and handling. (None of them are best in everything, and you know better than us which trade-offs you are comfortable with.)
But in terms of low-light capabilities, now you know what to look for.
 Comparison of sensor sizes: http://www.dpreview.com/learn/?/key=sensor%20sizes