It depends on the type of "speed" increase you are hoping to achieve.
If you are playing fairly recent titles at 1024x768 or lower with no AA, you'll likely see little improvement and possibly even some decrease in performance. I know I've noted quite a few Direct3D titles that play slower on my P3-800EB with the GF3 versus the trusty Ultra. It has a lot to do with hardwired T&L D3D games and driver efficiency. The GF3 drivers are substantially more CPU hungry in this respect. When you upgrade later in the summer, this will be a moot point though.
If you are talking higher resolution (1280x1024 and up) and/or adding HRAA, you'll see an improvement, except for maybe flight sims and the like.
If you are planning on plunking down cash for an upgrade, you may want to just simply move forward the system upgrade sooner then pick up a better matched videocard after that time. The majority of newer games are very taxing on the whole system and there isn't a single-bullet theory videocard that will help in these conditions on a low end P3. The advantage here is there will be more/better videocards to choose from at that point, and even if you stay with your original choice, the price reduction will be uncanny.
Cheers,
-Shark