I will recommend Globaltec in Santo Domingo. They are a small shop, but really good prices, good service, and they sell computers that they have built. They normally have several diiferent set-ups at a good price as a package, with warranty. They also will sell just components if that's what you want.
They are close to the UASD. If you come down Ave. Maximo Gomez from 27 de Febrero in Santo Domingo, go one block below the McDonald's restaurant, turn right at the next street (above the National Supermarket,) and go two blocks west. They are on the far right-hand corner of that intersection. (this is the cruz of calles Ramon Santana y Elvira de Mendoza.)
I think, but am not sure their Tele: 809- 687-3653 or 689-2387
In May, they had this machine for RD$12,900: CPU Intel Pentium 4 2.8 Ghz, 256 de memoria DDR400, Disco duro 80 Gb 7200 rpm, Quemadora de CD 52x, Video 16 exp 64 MB, 6 puertos USB Multimedia, Modem 56 BPS, Teclado and mouse, Juego de bocina (monitor no incluido).