GPU Is your graphics card.
Depending where you shop you'd be able to find a good PC for under $600 USD that can easily run 8 silkroad clients.
But of course, building your own computer (easy) is the best option, its cheapest and more bang for your buck.
It seems difficult at first but trust me its not, its just like putting together a table, just follow the darn instructions and you'll be done.
This is a monster of a PC for only $570 AMD Athlon II X4 635 2.90GHz, 4GB DDR3, 1TB HDD, DVDRW, Windows 7 Home Premium 64-bit
SRO isnt the most graphic intense game there is so you don't have to go above and beyond for all the parts you get.
heres another one:
Intel Core 2 Quad Q8300 2.5GHz, 6GB DDR3, 1TB HDD, DVDRW, HDMI, eSATA, FireWire, Windows 7 Home Premium 64-bit
For this one the GPU would have to be replaced but for less then $100 you can get a very very good GPU.
And a few more monstrous PC's that can easily run SRO with decent GPU's:
I'd always prefer nvidia graphic cards over ATI but ATi is just as good right now.
These are just my suggestions, Good luck!