It really depends on your monitor situation. 1080p? single monitor? 60hz refresh rate (standard)? In that case, the 550ti is "okay", about the most you'd want to upgrade it to is a 7870 or 660ti, anything higher is generally made for either A) Higher than 1080p B) 120 or 144hz monitors C) Multiple monitors. Something like a 7870, ~240$USD has been showing scores in benchmarks (linus's benches) beating both the 660 and 660ti (300$usd). It also comes with a 256bit arch, and 2gb video memory standard, this should give you better overclocking headroom than the 192bit 1memstandard 660ti, provided cooling is sufficient. Probably the most reasonable upgrade for a single 1080p display @ 60hz, is a last-gen 6870 or current gen 7850, both are very affordable and a solid step up from the 550ti.