in january, i ve started playing multibox x5

currently using a single pc assembled by me (a couple of years ago):
Intel dual Core duo E6300
MB intel 975xbx
4 GB RAM Corsair
Video card Nvidia 9600 GT VRAM 512 MB (added recently)
Dual Monitor 20" Samsung
Windows XP pro 32bit
ISP connection 20Mbit-2Mbit Adsl (virtual speed, never got + then 7/8Mbit down and 1.5Mbit up)
Little Home LAN 100Mbit
KEYCLONE software

would like add at least 1+ pc to this system, and i have already:

- an old MB Asus+Pentium 945D (need buy ram, video card pci-e 16x, HDisk, PSU and case)+monitor 17"(have it).

-a Dell pc XPS 420 with dual core quad, 3GB Ram and Video card ATI 3850 VRAM 512MB + another monitor 22" Samsung (bought in november 08)

-a notebook apple MacBook Pro, Intel Core 2 Duo a 2,66GHz, 4GB Ram ddr3 , 17" monitor , Video Card Nvidia 9600GT M 512MB (june 08)

-a crappy ancient but working complete pc with Pentium 1.6 Ghz, 2GB Ram RIMM (LOL), recent ATI video card AGP (cant remember model but bought in last 2 years) that requires a monitor or move one from dual combo.

My boxing situation is :
ATM I m playing to complete my roster and current objective is to bring lvl 60 last dps groups then start play 5 shamans again (lvl 71 atm) and continue levels all the game ranged classes groups together with my rogues and tanks (3 pala+1 DK+1 warrior). So, currently, i dont need great performance, coz spending time boosting chars in instances ->lvl 60.
In future, i want play pvp and pve in new content, so need configure better my playing system(s).

My question is:

How can improve drastically FPS (currently its really bad in major cities, 8-15 fps, and Dalaran, 8-12 fps, on main) and improve play in BGs (get really bad fps in AV and a just little better in other BGs)?
Consider in 5 men TBC instances and earlier WOTLK Nexus and UK fps and pc performance are decent .
I m thinking try to use at start 2 pcs, my assembled one and Dell xps system, with 3 or 4 monitors.
I will get better performances?
Or have to plan a full 5 pcs configuration?

Ofc I would like use in great part hardware i already have

Thx in advance for any suggestions