I can't, it is in the max and the launcher only leaves me to put up to 1 GB, I think this is because of my systen is 32 bits (Sorry for my english)java.lang.OutOfMemoryError
On the launcher click options and set more RAM to FTB.
Memory: 519571656 bytes (495 MB) / 899375104 bytes (857 MB) up to 1037959168 bytes (989 MB)
That's not enough!
You'll have to upgrade your hardware, unfortunately. Minecraft loads textures into RAM, from where they are moved into VRAM. If you are on a Desktop-PC you should be able to easily upgrade it. If you are on a laptop or Mac you will have to buy a better computer.
On the official Minecraft System Requirements page it is stated clearly that you need 2GB+ of RAM to run Minecraft and 4GB+ RAM is recommended. Soartex, Sphax etc. require a lot more of RAM and Video RAM, plus GPU power and CPU clock.
FTB and Vanilla are completly different. In vannila your computer only would have to handle the orginal Sphax texturepack and the shaders. However, in FTB it requires alot more processing power: it uses RAM to load mods, to load the Sphax textures + the Sphax textures for mods, then in-game it makes a lot of entities, new entities which will eat up more RAM... However, with 64bit I believe you will be able to play FTB with sphax.In Vanilla I can run 128x Sphax with shaders without problems.