I wish we could see all your specs on your PC MG...you said you're running a dual core but there's a fair gap in watt usage among models depending on the manufacturing process that was used for that model. My Core2Duo E8400 only uses 65W even though it's running at 3GHz whereas the Athlon 64 X2 uses up to 125W at the same speed because they were built with the older 90nm process (45nm is the standard now). It's little things like this that can make or break what you're trying to do here.
Dylan's spot on with his recommendation of Corsair as they use a well-respected OEM and 90% of the time their PSU's have one dedicated +12V rail (there's been a trend to split this rail, I've seen upwards of 4 of them FFS, but this has been knows to cause issues with older PC's).
PSU's are a bit funky in that not many of the brands actually make the internals themself, that's left to OEM manufacturers like CWT (ChannelWell Tech, one of the better ones) and others, so it can pay to do a little homework on who actually makes the guts for the PSU before you buy.
Let's put it this way, I'm only using a 380W Antec (EarthWatts model, that's the crucial bit because of who makes the internals in those models) in my PC right now and I'm running a Core2Duo (OC'd to 3.6Ghz), a Radeon 4850, 3 320GB hard drives (two in a RAID-0) and a burner...all on 380W and I have no hiccups, Crysis or no Crysis. PSU's are not unlike audio watts in respect to RMS vs. Max. Your home receiver or car stereo amp might say 1000W but it's only 650 RMS, almost half...once you hit 650, weird things can start to happen (ie. audio distortion vs. ripple/spikes/OS crashes in the case of PSU's). Total wattage is only one part of it but that's what they push on the box...it's the main selling point but not the whole picture.
But yeah, the main thing that can happen if you buy just the card is either your PC won't boot up at all or you'll get into Windows but when you start fraggin' away you'll either lock up or blue screen.
As far as waiting, GFX cards always go down in price (then back up, once they reach their lifespan and they know they can bleed people who don't have any other upgrade path, plus supply is running short because they're not making them anymore...same bollocks happens with RAM). With nVidia, the only reason the 275 is such a good deal is because of what AMD/ATI did with the 4800 series price/performance. Price drops are really going to depend on how their new cards stack up to nVidia's and how much better they perform to the last series.
Sorry for the long post...I try to explain myself the best I can so it's as clear a picture as possible.