I currently have a digital cable STB. The STB outputs at 576p by scart(RGB). I currently have the STB connected to the plasma by a scart-component converter(cypress), and the converter connected to the plasma using a component-VGA cable.

PQ is not good at the moment.
-lot of noise around logos, subtitles, etc
-video noise in the background(compression artifacts?/ musquito noise?).

Will a scaler help?

1) would i see a lot of difference/improvement using a dedicated scaler
2)is a dvdo HD+ enough, or should i go for a dvdo vp30
3) will i see a lot of improvement using the dvdo vp30 instead of the HD+
4) Do you know of another scaler, which will be better up to this task(but not more expensive than the vp30)

It is just for PQ improvement for SD TV. I already have a toshiba hd-e1 and PQ is amazing, don't need a scaler for watching dvd.

From what you are describing, it sounds like an Algolith Mosquito or Flea would be of more use than a scaler/deinterlacer. They specialize in noise and compression artifact reduction.

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I think this might be the problem.
The STB outputs in 576i, but the scart(rgb)-component converter is deinterlacing this to 576p, before sending this to the plasma. The rgb-component converter is a "cheap" 100 euro and probably a lot worse at deinterlacing than the panasonic ph9.Perhaps this could explain the artifacts and it would be better off having the plasma deinterlacing.
Could this cause the problem of the bad PQ and artifacts?