Need help in upgrading graphics card

As to your first question Yes

Second question depend on what Video Card you buy. The Video Card will tell you
the minimum amps on the +12v Rail of the power supply.

I didn't know if there were any particular types of power supplies. The one in my old system is more powerful than this but it's not compatible because I need different connectors

I can't see the link, sorry.

Assuming your case is a mid-tower, you'll just simply run a standard PSU.

If you want to, an HD5670 will run off of your power supply just fine.

you can always buy an adapter, many video cards even come with one.

Yeah, your link doesn't work. In general we would need to know things like your resolution and budget to make a card/psu recommendation.

I run at 1680x1050. Ideally I'd like to stay within $100-$150 for both, based on what I've been looking at I can probably do it for even less which would be nice

Not sure why the link isn't working, but I purchased the Recertified: HP Pavilion P6320Y(AY748AAR) Phenom II X4 820(2.8GHz) 8GB DDR3 NVIDIA GeForce 9100

HD5670/HD4850/GTS250 for 1680x1050.

You should be able to use the HD5670 on that PSU but it will be rather weak for your resolution. If you aren't the sort that particularly needs to use high settings/AA then it will be capable and allow all games to be playable.
The HD4850/GTS 250 shadow mentioned or an HD4770 are more appropriate cards for your resolution but they will require a new PSU. This HD4770 may be your best choice considering your budget;
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
It's $105(when you apply the promocode) but there's a $30 rebate so in the end it will only cost you $75. Alternately an HD4850 can be had for $107 initially and $92 after;
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
It's a slightly better card but uses more power.
For a PSU either of these are very high quality and should handle any of those cards;
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

im getting the same system,same specs,i do have a nvidea geforce 8500 gt with 512 ddr2,it will run on the psu.

my question is this,can both the onboard 9100 and my 8500 be run like sli (nvidea kinda hinted to that on their phys-ex page) or will i just have to disable the 9100