Since I just ordered one of these I've been researching the AGP vs PCI issue.
In my experience the only things that I am likely to upgrade are the video, memory and hard drive. It's clear that I can upgrade that memory and hard drive, but what about the video????
Unfortunately these systems do not have an AGP slot so upgrading the video will require a PCI video card.
Traditional wisdom preaches that AGP is a "must have" for serious graphics. While it is true that the AGP bus does kick PCI's but, the bus is not the bottle neck in most games. Even the fast AGP bus is too slow to effectively use a computers memory for textures; most proccessing these days takes place on the video card.
The above theory is demonstrated in a nice PCI/AGP comparison at:
http://firingsquad.gamers.com/hardware/v44500/
The review compares (among other things) the Voodoo 4500 AGP and PCI. In all gaming benchmarks, the AGP and PCI versions of the cards performed almost exacly the same:
"Comparing the AGP and PCI versions of the card against each other, there really isn't much of a performance difference unless you are running a high-speed processor. Even then the performance difference is still fairly slight. "
Unfortunately the lustre of AGP has lured most vendors into dropping PCI cards from their product lines. If you plan on upgrading the Dimension L series you are limited to PCI cards and that means you are limited to one vendor, 3DFx.
Now for an old man like myself this is not necessarily a bad thing. True Nvidia kicks 3DFx's but on every benchmark except those involving FSAA. However there are other considerations. I have a fairly large library of Glide games, games that would be useless on an NVidia or ATI graphics card. Also there are still games coming out today that play better on 3DFx hardware. A couple years ago 3DFx was king, and some of today's games, such as Deus Ex, are optimized for, and play better on 3DFx hardware.