Here's the thing.
A lot of people poo poo eMachines as cheap pieces of crap. But one thing they are not is loaded with a lot of proprietary components like a Dell would be. In most of their stock off the shelf configurations they are not barn burning speed demons. But they do look nice if you like mini-towers, the cases are good steel construction, won't cost you your first born son, have decent documentation and you can work on them. The re-furb deals on these with the mouse, KB, speakers, basically everything but the monitor and Vista loaded isn't too bad. In the case of this particular series, it is even upgradable to a reasonable extent.
Granted, it won't be the latest whiz bang mega gamer, but with decent graphics and a good CPU it is going to hold its own. It is worth noting that in the past there were some problems with the MBs, especially the TRIGEM boards in the older t2xxx P4 series, but lately they have gotten a lot better, albeit inexpensive, with the equipment.
On this particular model there is a wealth of information.
From the eMachines website:
Product description
Motherboard specs
T3656 note that it uses an ECS 945GCT-M3 MB.
Here is some more info from
The Unofficial eMachines Tech Info site:
Specifications:
ECS 945GCT-M3 (V3.0) Mainboard.
Processor Support:
ECS 945GCT-M3 (V3.0)
Manufacturers Specifications from ECS Elitegroup:
ECS 945GCT-M3 (V3.0)
From CPU-Upgrade another CPU support list with cross references to CPU part numbers and specifications.
CPU support
ECS 945GCT-M3(V3.0)
So the question is what upgrades would I do?
PSU - 450 watts + 80% efficiency minimum, this will depend on power requirements for the GPU. (the 250 watt in the stock unit doesn't have the juice to run much more than the stock configuration)
CPU - Fastest Core 2 Duo that I could afford. Just about any of these is going to be better than the Celeron.
GPU - Something decent in PCI express 16x. Amount spent here depends on how far one wants to push the graphics capability within the limitations of the case to handle the card size. Mini-towers are tight for space.
RAM - Throw another stick in to max the memory.
To reiterate, it wouldn't be bleeding edge super whiz bang fast. You probably can't BIOS overclock it and you are limited to socket 775 tech. It can however be a reasonably quick, solid platform and it will run Windows 7 nicely. YMMV, but I would play with one of these just to see how far I could push it within reason.
How far you want to push it is up to you and your pocketbook.