I recently upgraded my PIII 600Mhz 256Mb RAM, 32Mb TNT2. I started with the card too - got a 64Mb GeForce 4 440, and games played a lot better. But it wasn't until I changed the motherboard, processor and memory (now Athlon 2000 with 256Mb DDR RAM) that most games got that extra bit of speed.
Is your AGP slot 4x? If so, is it set right to use this higher speed? You will need to have a look in the BIOS (look in the mb manual for guidance if needed).
How much RAM do you have? My guess is it will be 100Mhz speed too? Games like FIFA use large textures and high quality sound that needs to be loaded from hard disk into memory, a process that is greatly speeded up by quicker memory, quicker buses, quicker hard disk speeds (7200 rpm and ATA 100 or 133).
I'm sure you will have noticed a large difference from your old card anyway, but if you wish to get the best you could invest in a few new parts. I spent £160 on the processor, card, memory and new case, so it's not too expensive.