I would take a look at the AMD 7770 and 7750, they go for about $140 and $100 respectively. There's no point getting too powerful a card as it would be bottlenecked, but I suspect you will be limited by power draw regardless. AMD have decent low-midrange cards, and they have been doing cards supporting 3+ screen capability for a while now, so they're a solid bet. The 7770 is quite a bit more powerful than the 7750, but it's $40-$50 more expensive so you'd have to decide whether it was worth it for your system. To be honest I suspect it wouldn't, however if you do decide to run three screen down the track it might be worth it.
There are a couple of caveats. You'll have to check what power connectors your power supply has. Cards these days tend to take at least one 6-pin PCI-E cable, which your PSU may not have. If it doesn't, that just means you're limited to 7750 or something like it which doesn't need any extra power connections.
Secondly, to drive three screens you have to output one of them from DisplayPort, which means either use a monitor with DisplayPort, or buy an adapter. And not every adapter will work, it has to be active one, which will probably set you back $40-$50.
Hope that helps.
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