I have the latter. 1x512MB and 1x256MB and yes it has a AGP slot Dell
Dimension 4400 Series
GenuineIntel
Intel(R) Pentium(R) 4 CPU 1.70GHz
1694 MHz
100 MHz
If you're looking for an upgrade on the cheap, you can go for some very well budget priced systems based on Socket 939 AMD CPUs, provided you're content with the level of performance it will provide and not looking to upgrade again in the not too near future. Judging by the fact that you're still using an RDRAM based 1.7 GHz P4 system, it seems like you'll be keeping your next PC a while.
A cheap upgrade would be any mid-level S939 Athlon X2, like a 4200 and Asrock S939-Dual SATA II mobo, and this combination will serve you well for at least the next 2 years, not to mention the Asrock motherboard has an expansion board to accomodate AM2 CPUs and DDR2 memory... of course, if you're willing to spend just a little more, you could also go Core 2 Duo, it's all a matter of how much you want to spend.
If you really must go with a newer video card alone, I believe NVidia as a 7600 series GeForce for AGP, but this will quickly go the wayside. If you choose to go the upgrade path and go with PCI-X graphics, the Radeon X1900 XT 256, for the money, is the best right now, period.