It's fine to have two different sticks of ram installed, you just won't be running in dual channel mode (which you weren't already, so it shouldn't matter)

As for the PCI cards, you're not going to get great performance with any of them, but of those the 5500 is your best bet. (EDIT: NOT, READ BELOW) I believe that the best pci card was the 5700, I'll look around for you.

ps the second one is the worst

Update, 5700s are as impossible to find as they used to be, and after some research I think you should actually go with that first 9250. Check out this review of an AGP 5500
and this comparison review of many 9250s

I'll keep looking on newegg for you.

Update 2.

If I were you I would get that Sapphire card (edit: read below), just because I would feel better about the price and the fact that it's a pretty decent GPU, whereas the 5500 is nortorious for being a dissapointment. It's also good that it's a 128bit edition and a decent manufacturer. Make sure you don't skimp out at the last second and get a 64bit edition, you can see in that review that it's a much crappier card.

Update 3.

OK so I actually read the comparison review, and it seems that the sapphire was the worst of the bunch, but as the pcb on newegg is black I'm assuming it's an updated edition, so it should be fine. If you have the cash it would seem that this GeCube edition is a better card, and don't worry about that user review, just remember never to install drivers from the CD in the box.