At this point you're out of luck. Any upgrades to that system will cost way too much money for too little gain: If you get a new motherboard, you'll find that it may not be compatible at all with your current processor or memory. Most new motherboards require a faster processor FSB than that of your current processor. And if you cannot set the motherboard's FSB speed to match your processor's lower FSB speed, you will have serious problems. And newer motherboards also require DDR2 or DDR3 memory, neither of which is compatible at all with DDR1 motherboards like yours. The cost of a new motherboard, processor and memory will leave you with no room at all whatsoever in your budget for even a decent low-end graphics card.

Moreover, even if you get a new motherboard small enough to fit inside your Dell's case, you may find that its mounting holes may not line up properly with the holes in your Dell's case. This can result, once again, in major problems if the mobo is allowed to rattle around in an ill-fitting chassis.

And even if the Dell's motherboard form factor and mount are "standard," you will find that your system's power supply unit may not be robust enough for an upgraded graphics card. As a rule, OEM system's power supply units are chosen such that it is barely strong enough to handle the original components which originally came with the system.

Put them all together, and you will find that the upgrade for your current system will cost more than double your maximum budget -- closer to $300.