I guess it depends on the rest of your system and exactly what games you play as to how much performance increase you will see. Same with whether it is worth upgrading. How are your games playing now? <A HREF="http://www.ixbt.com/video2/over2k4-ut-1024.shtml" target="_new">UT2003 at 1024x768</A> you can get about double the framerates of the MX440. The 9550 128-bit still shows a decent boost also. Not to mention having a DX9 card(although slow one) vs. a DX7 card. I tend to think it's not a bad upgrade.
Now, what are your prices? Make sure you are getting one with a 128-bit memory bus not 64-bit. Online in the US, there isn't much of a price difference jumping up to a 9600 Pro. Newegg has 9600 Pro's starting at around $100, while 128-bit 9600's are not that much cheaper. Unless the passive fan is something you desire, then the pro will give alot better performance. I understand you said you are not interested in the pro, but I'm doing you an injustice not to at least mention pricing similarities of these cards. For the price of a 9550 at a retail chain, you could probably order a 9600 pro online. If $105 is not stretching the budget too much, order a 9600pro. If the budget is $75, you should be able to find a 128-bit 9550 for that price. 9600's will run $80-$100 depending if you want a name brand or not. if you are outside the USA, sorry for mentioning prices as yours are probably different. Still no matter where you are, it makes sense to consider stepping up if prices are almost the same.
MSI K8N Neo2 platinum, A64 3000+, 1GB Corsair XMS 4000 Pro Series, Radeon 9800 Pro