In short, Yes it should work. Now I am taking it you are upgrading by buying the gtx 465 and you already have the gts? If so I might be a little concerned with their compatibility. Just because they are different platforms. I know from xp that the 500 series works with the gts series but not sure if the 400s do. They have issues with sli'ing properly with cards that are not of the same line or even number for that matter.
This might not be a problem though if your just running them separately though idk. Still in theory it should work, straight up.
If both slots are utilized, they both should run x8. However, for 465 as graphics it would hardly matter. It won't matter for any physX card also. Only very high end cards like 580 may see some performance drop. You shouldn't notice any difference on your setup.