With a Jtag you will need many key requirements:
1. Good soldering skills
2. The JTAG bundle itself
3. Dashboard version 2.0.737.1.0
Linky:
http://www.tiaowiki.com/w/How_to_JTAG_XBOX_360
Yes you can basically run games on a external HDD seeming as though you now have an xbox that can run unsigned code.
To be honest it isn't worth the hassle
As for the RRoD; don't waste your time with xclamp fixes, if done well they may give you at most 6 months - 1 year extra from what i can tell - the more full proof method is to reflow the whole motherboard
Seeming as though its just RRoD it still has some life in it, if it were the good ol e74 error then i highly recommend you just use the xbox as a door stand.
Good luck to you.
This isn't a walk in the park, not something one should do without the resources and the knowledge; i heard there are these USB launchers now, they take advantage os some glitch or something, it seems to have problems with XGD3 games; and its costly ~ 100 $ ish
Don't advise it.