There won't be any limitation at all unless you get a dual GPU PCB such as the 5970
Otherwise there aren't any cards that are really exhausted performance wise by running PCI-E 16x 1.x
I usually cite this.
techPowerUp :: AMD Radeon HD 5870 PCI-Express Scaling Review :: Page 25 / 26
Because PCI-E 8x 2.0 has the same bandwidth as PCI-E 16x 1.0
As you can see the 5870 gets 99% of it's performance on the 8x lane
The 5870 uses 153.6 Gb/s of bandwidth, so as long as you get a card at that level or lower, having PCI-E 1.0 shouldn't be a limitation as long as it's an x16 slot.
You can check the bandwidth of cards and many other juicy tidbits on these links.
Comparison of AMD graphics processing units - Wikipedia, the free encyclopedia
Comparison of Nvidia graphics processing units - Wikipedia, the free encyclopedia
Also, my card suggestion for your price range is a 4670 or 9600 GSO.
If you don't mind recertified cards...
That's a good deal.
Also if you get one of those cards, make sure it usees GDDR3 VRAM for maximum performance.
Also there's a 3850...
For $75 and less there aren't a whole lot of newer offerings that look worth it...
Considering the 4670 is still around.
The 4670 is the Playstation 2 of graphics cards.
It's old, many people still love it, it still can give you a **** of a lot of fun, and it's really cheap.