If you have sufficent RAM in your computer (2 GB should do) you should get between 8 and 18 FPS with that card, with an average of about 13 FPS. This is based on an old chart that LL posted in the blog back in November 2007, and assumes you are using the client and recommended system specs that were in effect a year ago. The chart can still be accessed, here:
http://blog.secondlife.com/2007/11/15/typical-frame-rate-performance-by-graphics-cardgpu/ .
Doubtful that you'll ever get over 20 FPS, even if you are alone and in an empty class 5 sim. By today's standards, that is a fairly low-end video card.
The GeForce FX 5500 was considered an "entry level card" when it was introduced, in 2003. A 5 year old video card is not a very good choice for running a real-time 3D application like Second Life, where the large amount of user-created content means none of the scenes can be pre-rendered. In a game like Counter-Strike, all the content is designed by the game maker, and every possible angle that you are allowed to look at anything is known in advance. So they can pre-render a lot of that game's images, to speed up rendering time.
To get even close to the 15 to 18 FPS range, you'll need at least 2 GB or RAM, and you'll need to turn down most of the rendering options in your Preferences. Things like draw distance, particles, object detail... It won't be as pretty, but you won't be wading in mollasses, either.
Look at Chosen Few's reply in this post :
/327/4e/282186/1.html#post2149290 for someone else who has a similarly old card, and is trying to get their computer to run SL adequately. He has some very good links to listings of better video cards for older systems.
By the way, I can feel your pain here, in that one of my two systems that I use for SL is a 5 to 6 year old PC that only supports an AGP type video card. It is so old it can't use the newer PCI bus cards, and originally only had an integrated video port on the motherboard. I bought a used NVIDIA GeForce4 Ti 4400, and 2 GB of ram, and it helped a lot. Your card is marginally better than what is in my old PC now, so with more RAM, you should be able to push into the 15 to 18 FPS range, as long as teh sim isn't too crowded.
Forget about busy clubs though, unless you like seeing slide-show frame rates. If you like the club scene, you need a better video card, and probably a better computer too.