Is it the Dimension 4400 you currently have, or that you are looking at?
All the 4400 reviews I've seen show it as having a Geforce 2 MX card, not anything as outdated as a Rage128 - unless it's a very early model, or the card has been changed.
However, it does look hopeful that it has a card in an AGP slot (is the monitor connector on a SLOT, and not on the motherboard backplate with the keyboard and mouse etc.).
The Rage 128
PRO is the first of ATI's 1.5/3.3v capable cards, so with luck, it will be a 1.5v slot that will not limit you (if it also supports AGP 4x mode, then it's 1.5v) - older motherboards with a 3.3v only slot (AGP 1x/2x mode only / AGP 1.0 specification) may not be able to install current cards (no 3.3v support - AGP 3.0 - 4x and 8x ONLY), or may have other problems.
AGP compatibility:
Motherboards:
1.
AGP 1.0, 3.3v only, 1x and 2x speed - face growing limitations in supported cards - will run only 3.3v capable cards, and not all of them!
2.
AGP 2.0, 1.5v and 3.3v, 1x/2x/4x - will run older AGP 1.0 cards, AGP 2.0 cards, and AGP 3.0 cards in 4x mode
3.
AGP 2.0, 1.5v only - Cannot install older 3.3v AGP 1.0 cards - the keyway should block insertion
4.
AGP 3.0, 8x - the majority of AGP 8x motherboard do not support 3.3v, only the 1.5v of 4x cards, and the 1.5v with 0.8v signalling of 8x mode
Cards:
1.
AGP 1.0, 3.3v - a really old generation, typically DirectX 6 level hardware, no TnL support, cannot fit to modern motherboards
2.
AGP 2.0, 4x and 1.5v - most are backward compatible with 3.3v, but not all will actually work in an AGP 1.0 system
3.
AGP 3.0 universal - have both 1.5 and 3.3v keyways, and should work in older systems, TRUE for a Geforce 440 MX - 8x, and common on older 8x cards or 8x capability added to low-midrange 4x cards.
4.
AGP 3.0 - only capable of 8x and 4x fallback mode, will not fit 3.3v slot.
http://www.ati.com/support/faq/agpchart.html#Figure2
Even if you don't have an ATI card, this illustrates all the possible supported/unsupported configurations, other than those that fit but don't work, but that's a chipset issue, rather than a slot compatibility issue.