I am in need of some good advice on GPU cards for my mac pro

[luckydog23]

I am in need of some good advice on GPU cards for my mac pro (early 2009) v4,1 for a 3 Monitor set-up (no extra power supplies please).

I am currently running Snow Leopard v10.6.8 but will be installing El Capitan v10.11.6 on a separate drive soon.

Right now my video set-up is as follows all utilizing 3- 21" Samsung 213T Monitors:

ATI Radeon HD 4870 512GB (MAC OEM) 150w - Running:
CENTER Monitor DVI (this is my main monitor for graphics and video work)
RIGHT Monitor MAC Mini DisplayPort with DVI adaptor (used for mostly graphics image review and file searches)
NOTE: this card utilizes both spare 6 pin power outlets on this box

Nvidia GeForce GT 120 25watts - Running:
LEFT Monitor DVI (usually used for overflow windows and open e-mail)
NOTE: no additional power outlet required

1) Presently I hoped to avoid any video card upgrades but would like to know if any good upgrades to my configuration would show noticeably achieved performance with plug & play ease? - OK I know this is asking allot but is it possible - I have to ask

http://pc4u.org/best-monitor-for-macbook-pro-retina-mac-mini-macbook-air/

I will need to be able to see the boot screen as well simply because I will be booting in different OSs (Snow & El Cap)

2) In Snow Leopard I have been noticing in my over-worn Safari and Firefox (both un-updatable versions) that the browser window scrolling has been having "slow down & hangup" issues when running pages with embedded video or other active elements. Could these just be conditions of the outdated browsers and will this be cleared up in the new El Cap OS or could there be a more sinister graphics hardware deficit/wear-down in play here that would require me to upgrade my graphics cards?
Will El Capitan require new cards anyhow for performing properly?

3) Is it more efficient to run both side monitors on the GT 120 (DVI + Mini DisplayPort) and utilize the full power of the ATI for the center monitor where most of my critical work is done..meaning do cards that support multiple monitors like the one I have perform better running just single monitors?

Any suggestions on UPGRADED performance configurations for this setup utilizing new graphics cards (ether 1, 2 or 3 cards) that will work well with limited issues and be kind to my power supply would be most appreciated.

Thanks in advance for any advice you can offer.

[TheOldMan]

Just to point out that this forum is a HP users forum.  Better help might be forthcoming in an Apple forum.