The Optiplex GX260 is limited in it's graphics card upgrading, due to,
1) The limited size inside the computer case, if the Optiplex GX260 is a Small Form Factor, (SFF), or Small Desktop computer.
The Small Mini-Tower may also be fairly limited because of size constraints.
3.57 inches Tall (9.0cm)
12.54 inches Wide (31.9cm)
13.93 inches Long (35.4cm)
4.2 inches Tall (10.6cm)
15.3 inches Wide (38.9cm)
17 inches Long (43.2cm)
16.7 inches Tall (42.5 cm)
7.1 inches Wide (18.1cm)
17.6 inches Long (44.7cm)
2) Uses AGP graphics cards.
However will use either 4X or 8X AGP graphics cards.
(AGP 8X being the better technology)
3) Power Supply
It's a 200 Watt supply, probably with a 10 Amp, 12 Volt power rail.
With one of the more powerful AGP 8X graphics cards out there, you probably will be alright, but the age of the power supply is what concerns me.
With the added load of an AGP 8X graphics card, it could cause the power supply to fail. Perhaps not right away, but probably right in the middle of a killer game.
The motherboard chipset is an Intel 845G.http://en.wikipedia.org/wiki/List_of_Intel_chipsets#Pentium_4_Chipsets
The motherboard chipset is the Northbridge, and Southbridge chipset.http://en.wikipedia.org/wiki/File:Motherboard_diagram.svg
(CPU stands for Central Processing Unit, or Microprocessor, or Processor)
When dealing with Integrated Graphics such as is on your motherboard now, the Southbridge chip handles it.
(Chip, and Chipset are slang terms for I.C., or Integrated Circuit)
The Northbridge chip on the Optiplex GX260 is an Intel 845G. The Southbridge chip is an Intel ICH4.
Just about any AGP 8X graphics card, will S-M-O-K-E the integrated graphics you have now.
Also Integrated Graphics has to use some of the System Resources.
Some of the Processor, and Ram Memory resources.
With a dedicated graphics card, it has it's own Processor, and Ram Memory.
(GPU - Graphics Processing Unit, and graphics ram memory)
This frees up the System Resources for the computer, and allows better graphics.
The SFF, and Small Desktop GX260 will require using a half-height graphics card. Not as tall as a normal graphics card.
The Small Mini-Tower may also. The computer case size is measured at the Outside edges. This doesn't give you an accurate measurement, of Inside the computer case.
For this type of GX260, you will have to physically measure from the AGP graphics expansion slot, (The small black slot above the white PCI slots, and below the Processor socket), to the inside edge of the computer case.
Determine whether the graphics card has to be half-height, or full height.
In an added comment I will list three AGP 8X graphics cards examples.
One in a half-height configuration, two in a full height configuration.