jimmysmitty :
It lookls like from what I can find, the best you can support is a E8400. I haven't found anyone with that DT nd a quad core.
You could try this one:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115058
Its a 65w quad core thats pretty decent performance wise and for L4D/TF2 at least would be a big benefit.
"Offically", you can go up to an E8500, this is hte list form Dell:
Subject: Optiplex 330 CPU Upgrade
D923J Conroe E8500, 3.16GHz, 6MB, WFD, E0
H399J Conroe E8400, 3.00Ghz, 6MB, WFD, E0
MP628 Conroe E6850, 3.00Ghz, 4MB, 1333FSB, G0
MP162 Conroe E6750, 2.66Ghz, 4MB, 1333FSB, G0
PN416 Conroe E6700, 2.66Ghz, 4MB, 1066FSB, B2
KU345 Conroe E6600, 2.40Ghz, 4MB, 1066FSB, B2
MP160 Conroe E6550, 2.33Ghz, 4MB, 1333FSB
DX484 Conroe E6420, 2.13Ghz, 4MB, 1066FSB, B2
DX485 Conroe E6320, 1.86Ghz, 4MB, 1066FSB, B2
Y933C Conroe E4700, 2.60Ghz, 2MB, 800FSB, G0
CY593 Conroe E4600, 2.40GHz, 2MB, 800FSB, M0
MP159 Conroe E4500, 2.20Ghz, 2MB, 800FSB, M0
RX522 Conroe E4400, 2.00Ghz, 2MB, 800FSB, M0
HU587 Conroe E4300, 1.80Ghz, 2MB, 800FSB, L2
T944D Conroe E2220, 2.40Ghz, 1MB, 800FSB, M0
RX391 Conroe E2180, 2.00Ghz, 1MB, 800FSB, M0
YU563 Conroe E2160, 1.80Ghz, 1MB, 800FSB, M0
YU562 Conroe E2140, 1.60Ghz, 1MB, 800FSB, M0
Y934C Wolfdale Dual Core E8500, 3.16Ghz, 6MB, 1333FSB, 65W, C0
Y935C Wolfdale Dual Core E8400, 3.00Ghz, 6MB, 1333FSB, 65W, C0
Y932C Wolfdale Dual Core E8200, 2.66Ghz, 6MB, 1333FSB, 65W, C0
X198G Wolfdale Dual Core E7200, 2.53Ghz, 3MB, FD, 65W,M0
Unofficially, one person at least has gone to Quad Core Q8400 (95W), but be on top of keeping the temps within limits. You might want to try Q8400S which is 65W (but more expensive).