Here's how these cards are similar:
- both are DX10
- both support a resolution up to 2560x1600
- both 512MB of GDDR3 (256-bit) memory
- both support SLI
Here's how they differ:
- the GTX+ will cost you about $50-$60 more
- the GTX+ runs hotter
- the GTX+ takes up 2 slots (GT only takes 1)
- the GTX+ has faster memory and core clocks
- GTX+ has more CUDA cores (128 vs. 112)
- GTX+ supports 3-way SLI (GT only 2-way)
What would you like to use it for? Gaming?
How many available slots do you have?
Most importantly, the question of whether your PC can take either of these cards depends on what kind of power supply you have. The GTX+ requires at least a 450 Watt PSU while the GT needs at least a 400 Watter. I'd make sure to have some headroom (a true 500 Watt PSU would be ideal).
You should also make sure you have proper airflow and cooling to accommodate the extra heat as these cards are in a different class than your X1550.
On a side note: upgrade the RAM if you can. 2GB is a bit low for almost any application you would need such powerful video cards for, but it should work.