The 7500 will be a negligible upgrade to a GF2U. Of the new cards, I'd consider the GF4Ti4200 the absolute best deal, if it comes out at $200. I have a feeling it may retail $50 higher, though--being only $20 more expensive (from the prices at Tom's HW) than a GF4MX460 may make the latter card obsolete on arrival.
Do any of the games you play run too slowly for you? If so, then consider upgrading. If not, then wait until you need to upgrade. If I were you, I'd wait until the Ti4200 is at a price you can afford. Especially if you want the bugs worked out, as the GF4 is a reworked (read: new & improved!) GF3.
The GF2U is the same price as some GF3Ti200's because the fast memory is the most expensive component.