Which 500W psu is it? What do you have on the 12V rails?
Honestly you have alot of options. You could go with a 4850, 9800GTX+, maybe even a 4870. You don't really have a power hungry system. You could get away with alot of that PSU has 32A+ or so on the 12V rails
I would say Yes to the GTX 260. But I would stay away from the GTX280. They have a 6 pin and 8 pin connector as it pulls more power. I believe you can get a 6 pin to 8 pin adapter, but I would play it safe.
The GTX 260 requirements are a 500W with 36A on the 12V rails.
* PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
* 500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode)”
* Certified power supplies are recommended.
* 1GB of system memory
* Installation software requires CD-ROM drive
* DVD playback requires DVD drive
* Blu-ray™ / HD DVD playback requires Blu-ray / HD DVD drive
* For a complete ATI CrossFireX™ system, a second ATI Radeon™ HD 4870 graphics card, an ATI CrossFireX Ready motherboard and one ATI CrossFireX Bridge Interconnect cable per board (included) are required
* Double slot solution"
From EVGAs site about their GTX260/216
"Minimum System Requirements
* 1GB of system memory (2GB recommended)
* CD or DVD-ROM drive
* 100MB of available hard disk drive space for basic driver installation
* Microsoft Windows Vista. or XP operating system (Windows Vista. required for 3-Way NVIDIA SLI)
* PCI Express or PCI Express 2.0-compliant system motherboard with one vacant PCI Express x16 slot
* One vacant add-in card slot below the PCI Express x16 slot. This graphics card physically occupies two slots
* 500W PCI Express-compliant system power supply with a combined 12V current rating of 36A or more
* Two 6-pin PCI Express supplementary power connectors"
Like the others, I`d go for either the GTX260 or HD4870 1Gb NOT the 512Mb version.
It`s more a matter of what is around in your particular country, and at what price because neither card is that much better/worse than the other.