would this be a good upgrade from what I have?
Zotac GTS250 512 ddr3 256 interface PCIE is what I have and I found this on tigerdirect.com
PNY VCGGT5201XPB GeForce GT 520 Verto Video Card - 1GB, DDR3, PCI-Express 2.0 (x16), DVI, HDMI, VGA, DirectX 11, Single-Slot, Low Profile.
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The first number i.e. GTS 250, and GT 520 is the series number. The 500 series is newer than the 200 series. While it isn't always comparable because both companies tend to change things around slightly, you would compare the cards based on the second numbers. 50 > 20. The 50 being a low/medium range card, while the 20 is a low-end card. So even though the GTS 250 is an OLDER series, it is a more powerful card than a GT 520.
Don't take this to heart though, as shown with AMD they really like to change around the meanings of their numbers. Your best bet is to look for benchmarks to make sure a card is an upgrade.