well not bad at all. Remember, you cant compare a AGP8x GPU with the PCI-E pendants. Geforce 7800GS is the fastest card for AGP atm, even if its pretty expenisve.
A cheap upgrade might be a 6600GT 256MB, but it will not fit requierments for upcomming games well enough.
Depends absolultey on how much you´ll spend and what you expect of your card. A good 6600GT 256MB will be outstanding compare to your, sry to say so, shitty 5700LE...
Nvidia AGP cards listed form FX5 series to 7series (weakes first):
FX5200 Ultra, FX5500, FX5700LE, FX5700, FX5900XT, FX5700 Ultra, FX5900, FX 5900Ultra, FX 5950 Ultra, 6200LE, 6600, 6800LE, 6600GT 128MB, 6600GT 256MB, 6800GT, 6800 Ultra, 7800GS