Your CPU is going to be a huge detriment to anything you upgrade, just so you know this in advance. Especially if you're going to be playing the new WoW Cataclysm update.
You listed "highest possible" for monitor resolution. Do you wish to also buy a new monitor? If not, this answer is no accurate. You need to tell us the MONITOR RESOLUTION of your existing monitor. This way we can help you select a video card.
That being said, with such an old/slow processor you don't want to spend $150 on a GPU as it would be entirely wasted.
Also, is your current GPU an AGP card or a PCI-E card? [Edit: your link seems to indicate it uses a PCI-E GPU]
I know what you mean about my CPU.... sucks but, right now I can't afford to start from scratch and build a new PC, and I don't think I can replace my current stock CPU. I am just looking to see what other options are out there for GPU to reduce noise from my current one. it is way too loud. I use a PCIe... not sure if I can use 2.0 x16 or not though.
My monitor resolution I believe goes to 1280x1024 i think.
Any of the PCI-E 2.0 cards should be backward compatible to work with PCI-E 1.0 slots. So that shouldn't be an issue. I can't see having you spend any more money than the ATI 5450 @ $49 though. It would be a complete waste to do anything more powerful.