"Only 128mb ram". Hmmmm. This is kinda true, but not really. If you're talking about your system ram, then 128 is low.
However, for a video card, 128 is fine. I am runnin a ATi Radeon 9600 Pro 128 right now, with an AMD XP 2600 and I've never had any slowdown problems on games with reasonable settings. (IE at lease 1024, medium graphics, etc).
My advice to you: Wait until the end of summer if you can. NVidia just released its 6800SE card, and its a killer. ATi is soon to follow suit. After that, you'll be able to pick up an ATi Radeon 9800XT with 256mb for cheap. Around $150. (Thats cheap for the particular card, in comparison to like $400 now).
Its all about what you want to spend. But follow this.
If you're buying a card new right NOW, such as ATi Radeon or NVidia 5xxx series, stick with the ATi. NVIdia's run hot and hence have humongous fans on them which can make you system hard to modify if need be. (It actually blocks out the PCI slot below it.). And ATI's cards in the same class out-perform NVIdia's cards.
This can all change in the next few months though, as the new Nvidia 6800's are insanly powerful.