The Dell vostro 400 is a mini tower and there is not much room to expand. The originals could be configured with an 8800GT or an 8800GTS when ordered from Dell. The power supply is weak and will not support much for a graphics card. These are two issues I see you running into. It does have a PCI-E x16 slot which is great for a business class Dell.
1. Make sure you measure the space available for a new graphics card in side your case. Before you buy a card find the exact dimensions of the card.
2. You computer only has a 300w Dell power supply. I would recommend an upgrade with any new card you buy. You don't want to risk your computer to a 300w PSU...you might be disappointed later.
3. With a budget of $250 you can get a nice card and new PSU...that would be the best option.
Here is a link to PC Power & Cooling website. They specifically list the Silencer 500 as Dell vostro 400 compatible
http://www.pcpower.com/products/power_supplies/dell/
And here it is on Newegg for $60 after rebate
http://www.newegg.com/Product/Product.aspx?Item=N82E16817703015
With that said...I would then recommend maybe a 9800GTX+ or a ATI 4850 card for you. Both will fall in your total budget and they both game very well
Just make sure it will fit before you buy.