I think I'm going to get a Radeon 4670 video card, as it looks like it's going to give me the most bang for my buck. To upgrade any further means getting something like a 5770 and a new mobo/cpu, which of course means new PSU/Case/RAM/etc and costs quickly spiral out of my price range.
My questions are:
1) will my stock PSU support a 4670? It's a 300W with 18A on both 12v+ rails. If I need to upgrade, what's the least I can spend and get something reliable?
And somewhat related:
2) Should I get 1Gb DDR2 or 512 Gb GDDR3 with the video card? My monitor maxes at 1440x900, so higher resolutions are not needed. If the performance will be similar (my motherboard has PCI-E 1.0 btw), which one will be less of a power drain?
1) Yes, a 300W power supply should be fine. The manufacturer recommends 400W, but those numbers are always vastly exaggerated because they don't know what the rest of your system is. So to cover their asses, they assume you're running an overclocked quad core with five hard drives, a power saw and a mini-fridge.
The 4670 is designed specifically for low power consumption, and the PCIe version draw all its power through the slot, doesn't even need an external power cable. I've heard of plenty of people running 4670s on a 300W PSU, and if it makes you feel any better, I've got a 1GB 4650 running in my old machine, on a generic 300W power supply that's a POS.
If for some reason you need a new PSU, you can probably pick up an passable one for $25 or so.
2) Get the 1GB DDR3 card, which is what one of the ones you picked is. 512GB vs. 1GB shouldn't matter much unless you went to a slightly higher resolution, but for the same price, you might as well get the better one.
2a) It doesn't matter if you have a PCIe 1.0 slot, a 4670 still will not come close to using the entire bandwidth.
Also one says GDDR3 and the other says DDR3. The clock speeds are listed as identical. Are they both actually GDDR3? I've heard that they are different but a few google searches seems to indicate that they are in fact the same and it's just the result of nonstandardized nomenclature.