what's a good rendering program to be used with sketchup?

I'm not 100% sure what you mean by 'rendering program'. Normally, a rendering program in 3d graphics is something that's gonna take a scene you've created - complete with textures, meshes, and lights - and convert it into one or more images or videos.

Many CG suites bundle creation and rendering together, so that there is effectively no separation between the two (other than that you press a "I'm ready!" button to start rendering).

If you wanna really go all-out, you could look at the personal learning version of Autodesk's Maya, which is basically what everyone who's anyone uses in the CG industry (Pixar, most video game companies, etc. use it). I'll warn you that Maya, being an industry standard, has a particularly steep learning curve. If the difference between NURBS, polys, and subdivs makes you go "huh?!", you may wanna consider something else.

Perhaps clarify what you mean by 'rendering'. What's the end PURPOSE of what you're trying to do?

You could also use free programs like Blender, which would probably be easier to use than Maya.

EDIT: In response to the below, thanks, and my response:

The trouble with Sketchup as modeling software is that it actually does not produce very high-resolution models. For example, it does not (as far as I remember) include bump mapping or any advanced texturing/lighting techniques. Photorealism is a particularly difficult thing to achieve (our eyes are very good at spotting odd stuff), so it may be a difficult thing to achieve. So you could potentially import your models into Blender, but you'd have to learn how to do the lighting effects yourself.

Alternatively, you COULD use something like Podium, with the caveat that it is rather expensive (the trial version lasts only 30 days).

Vray, being profession rendering software, is even more expensive ($800 from one site).

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