The fastest is the X800GT. It's a tough call because this card while faster will also get outdated faster as it doesn't fully support the actual DX9.0c standard. (it's a 9.0b -SM2.0 only card) The 7600GS is not as powerful, but it's fully DX9.0c-compliant. So it depends how long you want to continue with this configuration. If you just want the card to play actual games as best as possible, get the X800. If you want to try the new games when they come out, get the 7600GS.
Check out if you can't find any X800XL or X850Pro, they're faster. Also the X1650Pro is slightly better than the 7600GS, if you can find it.
Check your power supply spec's before considering a replacement AGP card, because your PSU may be a limiting factor. The X850 Pro is a faster card, and some of them can be unlocked to 16 pipes, however the 7600 is more power efficient, doesn't require an additional 4 pin molex power connector, and is also a good performer.
With a 19" LCD you're going to probably have a native resolution of 1280x1024. That's kinda high for those cards. See if you can put a few more bucks into your card. Look for high framerate over features, because honestly at this range you're going to be stretching to play newer games on that monitor. Ebay is a pretty good source of x8** agp cards in the low $100s. Be careful buying new, though, as agp is currently industry code for "awfully grotesque prices". I'm talking sometimes 70% more than the same PCI-E card. You could get a new motherboard and a PCI-E 7600GT for less than just an agp 7600GT, which is exactly what I would recommend, really I didn't notice your PSU power, so you'd want to calculate that, of course. There are some handy stickies over in the PSU section.