Tom's Hardware VGA Charts Part V: PCI-E graphics cards
The 128-bit, 128Mb GF6200 and X300 cards actually run HL2 and CS:S at very playable framerates if you keep the resolution down to 1024x768 and keep FSAA and AF turned off. The basic GF6600 runs significantly faster than either of these cards though.
The GF6200TC and the X300SE aren't on the chart, and are supposed to be severly constrained by memory bandwidth. Expect worse results than the worst cards in this roundup.
If you're OK with 1024x768 and no AA/AF, then it's possible the X300SE may still be playable. You won't know unless you buy a card and play CS:S with it. These are the kind of bottom feeding parts that nobody ever reviews.
If you're going to step up from a X300SE to a real X300 or a 6200 128-bit card, you may as well spend the minimal difference to get the 6600 or the X600 Pro. Both are significantly faster (25% or better) when playing CS:S at 1024x768. You would be able to turn on AA/AF and still be well above the 30FPS mark.
New Egg has the X600 Pro at $99 and the GF6600 for $114, the X300SE is $61 and the X300 128-bit is $78. If you spend the extra $53 to go from the X300SE all the way up to the GF6600, you will be getting at least double the performance when playing CS:S.
You have to foot the bill, but each step up in cost will result in a noticable improvement in CS:S framerates.