PCI Express x16 2.0 vs 1.0

Hello all, this is my first post at Windows BBS (kinda a newb), just needing some information. Recently, my laptop broke and my its just so happened my parents ordered a new computer and gave me their old dell dimension 9150. So, i wanna try and get back into comp. gaming again.
The specs are...
4gigs of ddr2667 ram
Dual Core Intel Pentium D 805, 2666 MHz
1 pci express x16 1.0 slot
1 pci express x4
2 pci express x1
375W Power Supply

I've been googling the performance loss of using a PCI-E 2.0 card in a 1.0 slot, and found mixed results. I just want to try and get as much power out of it by getting a new graphics card and possibly replacing the power supply if i need to. I've been looking at newegg and found an overwhelming amt of PCI-E 2.0 x16 cards. I'm not looking to get the best graphics card by any means, whatever 200$ can get me. Can anyone give me any good suggestions on cards? Also, can anyone tell me how much performance might suffer if i did use a 2.0 card in a 1.0 slot?

 

Cool, thx alot I'll check it out!