If you're looking to up the ISO so you don't need to use a flash use ISO 800 to whatever the max ISO is on your camera. Depending on the lighting you may be OK but would probably need to use the largest aperture (smallest number) on your lens. The problem with this is a shallow depth of field and at higher ISOs you get noise.
I don't know what a "rps" flash is but your camera is capable of TTL metering and if your flash can communicate with the camera - I would use the flash. It will give you more control, use a lower ISO and you'll keep the shutter speed to a speed where you shouldn't get motion blur or camera shake if you don't use a large focal length lens. You just have to remember that a flash will expose people and objects that are close to you differently than someone that's far away. Heck, if your camera doesn't communicate with the flash I would use the camera's flash for all the above reasons. It's not going to be as great with the on camera flash but it will be better than camera shake and motion blur IMO.
I have the D90 and only had the camera's flash and have used it indoors with pretty good results, I would assume that the same would be true for the D5000. I also have used the D90 at ISO 6400 where I couldn't use a flash and the photos came out OK with some noise.