You simply need a quality capture card with SVideo inputs.
The standalone boxes tend to convert to DV on the fly, and have a habit of crushing the blacks and whites quite severely and giving relatively soft captures.
Hi-8 is similar to SuperVHS quality wise and has excellent colour resolution, so you really want to capture uncompressed via SVideo. You want to use SVideo as Hi-8 records the Chroma and Luma separately, so it makes sense to keep them separate.
The PDI Deluxe/Sweetspot card (internal PCI card) in conjunction with DScaler software for capture will give you pretty much as good as it gets results for Hi-8 capture.
For the best quality playback of Hi-8 via SVideo, the Sony industrial/broadcast decks are your best bet, but even their camcorders give pretty good playback.
It is impossible to say how good your captures look really as we can't see the source with our own eyes.