6600 by old school standards is high spec
and considering that most games run on the older hardware (GeForce FX 5950/Radeon 9800XT being the kings of that) the 6600 and 6600GT do seem rather impressive, given that they are 1.5x to 1.7x faster
if not the same speed with SM3.0
so the 6800 and X800 series look rather outrageous (considering how fast they are, I mean, seriously, just look at some benchmarks, and I can personally vouch for the 6800GT never dipping below 40 FPS, and I run all games at 1152x864 (Except a few old RTS games that don't support that))
aside from that
in the new naming scheme the 6600 is considered low-middle range
it's the 5600 of it's day
I would personally suggest a 6600 or 6200 for AGP
but you'll want to avoid most cards that have the power connection plug on their rear (as it means they consume upwards of 66W of power)
the entire NV35 and NV38 lines do require a lot of power
while providing comparativly low performance to modern middle class cards, which do not require so much power
for example
the GeForce FX 5950 Ultra requires 79W of power!
the 6800U doesn't even touch that requirement (it's at 77W -ish)
the 5900XT, 5900, 5900Ultra all of them require a lot of power and make a lot of heat
and they provide less peroformance that cards that require as much power/make as much heat
I could probably safley say that a GeForce FX 5900 Ultra is comparable to my 6800 GT in power use and heat creation, and the 6800GT is a little over twice the performance
so you have to consider which generation you buy from
usually as the cards advance their power requirements are redefined, and lowered so that they can get more performance out of the device
I would say 6600 or 6200
if you want a GeForce FX (which I do not suggest, given their age they will cost a fair ammount, while the performance is good, GeForce 6 offers more)
but if you really want a GeForce FX i'd suggest a 5500 or 5700
aside from that, if you wanted to go ATi Technologies, a Radeon 9600XT can be had for $100 (it's old gen middle range) and 9800Pro's for around $130 (and it's a HUGE step up in power) but a 9800Pro will require more power (you should be able to get by on a 350W unit, assuming you have a newer Dell (dell used to use junk PSU's, but has recently started to use PC Power & Cooling (in the last yr +/- a little)
a 9800Pro is an excellent overclocker toy
for $130 it's a good deal
with the exception that 6600GT's are $159
you can push a 9800Pro a 9800XT instantly (they are VERY close to each other in speed, so it's a simple OC of under 100 MHZ) and then go from there up a little
but that will create more heat/require more power
personally i'd say if you want a ton of performance
consider a 6600 or a 9800Pro
but the GeForce 6 card is going to have many many advantages over the Radeon