Is 16X worth it?

Here's my problem:
Gigabyte K8NS-Pro mobo w/Athlon64 3000+

and my other problem:
Nvidia GeForceFX 5200

I want to upgrade to a better video card but AGP 8x is the best I can do with my current mobo.  If I upgrade to a PCI-E 16X board, will I see a significantly better performance boost than using 8X?  I don't have unlimited money so I can only buy one thing, either a video card or a motherboard.  I already did a bit of research and I was thinking of getting a mobo with 16x and an integrated (bleh) Geforce 6150 seeing as how it performs better than my current video card and probably get a 6600gs maybe 2 months from now.

Summary:
AGP 8x now or PCIE 16x later?
Half(?) performance now or good performance later?

Two things:
a) It's not half.
b) There's always something better on the horizon.

5200FX is not a good investment

Go for a 6800GT or something equally current. It might be overkill now, but three years from now when you're still using it you'll be glad you spent the $250

Thanks for the comments.  I've decided to go with AGP for one last time.  Before I leave this topic to die, I need one more comment to decide which video card is the better of the two.  A Geforce 6600 vs. a Radeon X700.  I'm looking for features such as SM3.0 which would be a pretty obvious choice but I heard Radeons are better for eye candy.  So basically what I need is comparative benchmark scores.

BTW If you know of a website that does head to head video card benchmark brawls, let me know so I can avoid this topic in the near future.