I want to play Oblivion, andI have a Dell Dimension 4700 2.9GHz and 4GB RAM,

I just bought a Radeon HD4650, which was recommended by Dell, but I hear that it doesn't work as good as they say it does, so I went to UESPwiki and read the chart which puts playability at "very high"

Truth or Untruth?

That card should be able to play the game around high settings. Both the game and that card are a bit old. The 4670 should have been able to max the game back in the day. But those cards are 3 generations old at this point.

However your processor (what looks to be a 2.9Ghz Pentium 4) is also not helping matters. The game is very CPU bound, in fact more so than GPU bound.

If you just want to play Vanilla, that should be more than enough.

Modding will be a bit iffier, but still quite doable, within reason.